I did some testing with the Mono 2.2 release VMWare image, and I was able to produce a build of the latest mojoportal that works without the errors reported in this thread. To do this I had to change it back to use the 1.0 System.WEb.Extensions, as apparently the current bugs are all related to the 3.5 .NET version.
I've uploaded a new release package for Mono mojoportal-2-2-8-2-c-pgsqlreleasefiles-for-mono.zip
It does not have the errors reported about adding features and I even tested it with folder based sub sites using relatedsitemode and it worked fine.
Best,
Joe