What holes in paleographic record does this fill? If a graphic code were unearthed without any good contextual clues (eg, something like Vinca symbols), can we look at it and say this does not look like it has gone through a transmission chain that obtains with glottographic writing, because of x, y and z featur

Why we might be wrong

If no compression effects are found this may well be due to the fact that the Vai writing system may have already been optimally compressed at the time of its creation. Conventionalised but non-linguistic graphic systems were in use by the Vai people prior to their invention of writing (see Kelly [micenei]) and it is argued by literate Vai that elements of these systems have informed the design of the script (Massaquoi). If this was the case then the transition from a semasiographic system to a phonographic one was in itself a substantial "compression event" in which previously multi-valent signs were affixed to single syllables. Such a radical narrowing of interpretation, brought about through a conscious process of group deliberation, would already account for most of the compression to the system: subsequent transmissions would amount to only minor tweaking. A recent investigation has shown a distinct yet analogous finding with regards to a cardinality bias in the world's writing systems. Morin demonstrated that scripts do not tend to become more cardinal over multiple transmissions in historical time, but that cardinality is "baked in" from the beginning. Thus, we may discover that the near-optimal compression of Vai and other non-laboratory generated graphic systems was an outcome of upstream cognitive processes. 
This is not to invalidate semiotic transmission experiments generally, however a a negative finding may suggest that not all cultural items will change in the process of transmission. This would speak to the trade off between set and the item.
By contrast, it may also show that the system is not optimally compressed but that outside sources of inertia are acting on the system preventing compression effects from arising. Attempts to standardise the Vai script have occurred rarely and late in its history, while the script has not been taught in an institutional setting since the 1840s. As such, a major source of inertia in the form of institutional pressure, has been removed. If we detect resistance to compression then we would be unable to argue that such conservatism was a result of top-down institutional pressures (unless such institutions escaped notice in the documentary historical record). Unlike language, writing leaves a permanent impression and the dynamics of asynchronous communication mean that misunderstandings can not be repaired in real time (REF Topics paper). Thus, it could be the case that demands of successful asynchronous communication exert too much inertia on the system. Complex systems with high levels of redundancy still manage to get transmitted with minimal change (ref Esk). As a result of borrowing, languages sometimes end up with writing systems that are ill-suited to their phonologies but these can also remain successful over long time periods (Turkish, etc morphology group email). 
All experimental participants will already be literate in the Roman script and perhaps others. This means that the experiments and the historical data are not directly comparable on the dimension of literacy. Consequently, changes to the Vai input across experimental generations may be conditioned prior literacy in another linear script, thus items may become more Romanised over time. In itself, this does not undermine the premise since our study is interested in detecting compression regardless how that compression is actualised. 

Diachronic analysis of historical Vai script

A description of the data

Samples of the Vai script have been recorded and preserved from 1834 to 1981. The full comparison table with original images is available at [osf link]. For the purposes of this study, however, we have selected the 55 most frequent characters as identified by Rovenchak, Riley and Tombekai. In compiling the shorter 55 character dataset we made several decisions about the original data. Three of the consecutive sources were collected within a very short space of time, namely Koelle 1849, Forbes 1851 and Ndole ca.1850. In the dataset we excluded Koelle 1849 and Forbes 1851 on the basis of Stewart's observation that some of the characters were distorted in the process of their documentation. The Ndole material, meanwhile, is reproduced from a handwritten manuscript by a literate Vai and is thus taken to be more authoritative. 
Family resemblance.

Results

Lorem ipsum

Experiments using high-frequency Vai characters

Lorem ipsum

Results

Lorem ipsum

Discussion

Lorem ipsum

Acknowledgments

The following institutions and individuals helped us secure rare manuscripts for our dataset: Asien-Afrika-Institut (University of Hamburg), Hella Bruns (Max Planck Institute for the Science of Human History, Jena), Valerie Haeder (Library of Congress, Washington). This primary Vai data was tabulated by Olena Tykhostup (Freidrich Schiller University, Jena).