One of the problems with designing a constructed sign language is that so little is actually known about sign languages compared to oral languages. For many conlangers and projects (e.g., sign engelangs or loglangs, etc.), this isn't really a big deal, but it is a serious problem for the aspiring naturalistic con-signer, ascribing to the diachronic or statistical naturalism schools.
I have, however, come across one account of the historical development of modern ASL from Old French Sign Language. While it is hard to say if the same trends evidenced here would generalize to all sign languages, they do seem pretty reasonable, and provide a good place for con-signers to start. Additionally, it turns out that many of these diachronic tendencies mesh rather well with the goal of designing a language with ease of writing in mind.
Unsurprisingly, despite the relative ease of visual iconicity in a visual language, actual iconicity seems to disappear pretty darn easily. But I, at least, find it difficult to come up with totally arbitrary signs for things - much more difficult than it is to make up spoken words - and the Diachronic Method is generally considered a good thing anyway, so knowing exactly how iconicity is eroded should allow a con-signer to start with making up iconic proto-signs, and then artificially evolving them into non-iconic "modern" signs.
The general trends in this account of ASL evolution can be summed up as follows:
- Signs that require interaction with the environment (like touching a table top) either disappear entirely, replaced by something else, or simplify to avoid the need for props. That seems pretty obvious.
- Signs that require the use of body parts other than the hands for lexical (as opposed to grammatical) content tend to simplify to eliminate non-manual components. E.g., facial expressions may indicate grammatical information like mood, but won't change the basic meaning of a word.
- Signs tend to move into more restricted spaces; specifically, around the face, and within the space around the body that is easily reached while still keeping the elbows close in. This is essentially a matter of improving ease of articulation.
- Signs that occur around the head and face tend to move to one side, while signs occurring in front of the torso tend to centralize. This makes sense for keeping the face in view, especially if facial expressions are grammatically significant.
- Two-handed signs around the head and face tend to become one-handed signs performed on just one side. In contrast, one-handed signs performed in front of the torso tend to become symmetrical two-handed signs.
- Asymmetrical two-handed signs tend to undergo assimilation in hand shape and motion, so that there is only one hand shape or motion specified specified for the whole sign, though not necessarily place or contact. This is a matter of increasing ease of articulation (reduction how much different stuff you have to do with each hand), as well as increased signalling redundancy.
- Signs that involve multiple sequential motions or points of contact "smooth out".
- There is an analog to "sound symbolism", where, if a large group of signs in a similar semantic domain happen to share a particular articulatory feature (similar shape, similar motion, etc.), that feature will be analogically spread to other signs in the same semantic domain.
Symmetrization makes writing easier because you don't have to encode as much simultaneous stuff. Even though two hands might be used, you don't have to write down the actions of two simultaneous hands if they are doing the same thing. Reduction of the signing space also means you need fewer symbols to express a smaller range of variation in the place and motion parameters, and smoothing simplifies writing essentially by making words shorter, describable with a single type of motion.
Many two-handed ASL signs are still not entirely symmetric. Some, like the verb "to sign", are anti-symmetric, with circling hands offset by 180 degrees. One-handed signing is, however, a thing, and communication can still proceed successfully if only the dominant hand performs its half of the sign, while the other hand is occupied. (I imagine there is some degradation, like eating while talking orally, but I don't know enough about ASL to tell exactly how significant that effect is or how it qualitatively compares to oral impediments.) Thus, it appears that it would not be terribly difficult to make the second hand either completely redundant, or limited in its variations (such as symmetric vs. antisymmetric movement, and nothing else) to make two-handed signs extremely easy to write, and minimize information loss in one-handed signing.
Given the restriction of two-handed signs to particular places (i.e., not around the face), it might even make sense to encode the action of the second hand as part of the place. One could imagine, for example, a non-symmetric case of touching the second hand as a place specification (which would typically remain possible even if that hand is occupied), as well as symmetric second hand and anti-symmetric second hand.
I have no idea if native signers of ASL or any other sign language actually think of the second hand as constituting a Place, just like "at the chin," or "at the shoulder," rather than a separate articulation unto itself, but treating the secondary hand as a place does seem like very useful way to think for a con-sign-lang. Not only does it significantly reduce the complexity required in a writing system, it also ends up smoothing out the apparent surface differences between near-face signs and neutral space signs; in each case, there is underlyingly only one lexical hand, with the second hand filling in when the face is absent to better specify Place information.