Hi,
mojoPortal has it's own url re-writer so using one from IIS before it is going to be problematic. I recommend firts understand how our url re-writer maps friendly urls to real urls.
http://www.mojoportal.com/using-friendly-urls-in-custom-features.aspx
you can use extensionless urls as documented here:
http://www.mojoportal.com/extensionless-urls.aspx
some additional information I think may help in the videos
http://www.mojoportal.com/dev-series-7-concepts-part-1.aspx
and
http://www.mojoportal.com/dev-series-8-concepts-part-2.aspx
using some arbitrary extension may be possible, I don't know and I don't test that or try to support it.
Best,
Joe