You say that "Visual and aural enhancements take a lower priority." for your mod, which I personally find is the way to go.
However, in the long run I would suggest using
custom models and skins for the zombies, hopefully it's even possible, I'm no developer, (as you've noticed with my lack of information concerning certain subjects in context with development.) but I think it could really help the mod distinguish itself, even more so with the ever present originality of it's game play mechanics.
If in fact, you can or are willing, in the future to add custom models and skins I would strongly suggest these ones;
http://www.fpsbanana.com/skins/89287
If not something similar, and with the permission of the original author's consent which he will most likely be gladly willing to give you to achieve a common goal.
And also a question:
Does anybody here know if zf_asylum is still in development?