I did set the shared secret on Monowall and configured it as an access point in ZeroShell with the appropriate shared secret, but it did not work.
I looked into Monowall a bit and it appeared to use PAP.
The only thing unusual about my test is that the Monowall that I was attempting to use was on the other side of a firewall, so there is the possibility that something is amiss there, though I thought I had the firewall rules in place appropriately.
I think that I’ll just move my test Monowall on to the same segment as Zeroshell to rule out any firewall issues and try again. Having both Monowall and Zeroshell going after the same user database would be a good thing.