I am pretty sure this is also a Mono bug as it works on Windows. I've seen a few cases lately when testing mono where I get thread abort exceptrions and see a blank page, and this is even using the latest mono from svn. I know there was a bug fixed recently in Mono where the url-rewriter wasn't working correctly because on IIS it was re-writing to ~/Default.aspx but on Mono it was seeing just /Default.aspx and this was causing problems for virtual directory sites like localhost/mojo because it was not resolving the path correctly. This could be also causing the problem you are seeing.
I'll try testing the latest Mono again today, if I can get past the thread abort exceptions I was having I will be able to fnd out if the same errors are being logged when adding features to child sites.
Best,
Joe