It's documented in the mod_mono (8)
man page as follows:
"MonoIOMAP"
Takes alias name and a string of the same format and meaning as the MONO_IOMAP
environment variable (see the mono(1) manpage). The effect is that mod_mono performs
the same actions as mono runtime with MONO_IOMAP in effect, as well as that the
MONO_IOMAP variable with the specified value is exported in the backend environment.
This option is necessary when you're using MONO_IOMAP in your application and there
might be static files with mixed case names - in such case mod_mono attempts to open
the file directly and sends it using apache facilities.
Default value: none
The MonoSetEnv
should still work, however mod-mono itself won't act on that setting except to pass it on in the environment.