I duplicated your error here on my dev server. I added the mosso config file to root of my site, and the problem went away. I don't think all the hosting companies go for the full blown medium trust level, they use a modified version where they turn important items back on like reflection. I could be wrong.
I would set up a small demo of sitefinity on the live hosting enviroment to see what happens, if you have a sandbox on the live server.
You should see if you can get a copy of there web.config medium trust settings. Mosso provides one for Mosso users at http://cloudsites.rackspacecloud.com/index.php/Overview_of_modified_Medium_Trust
. The page has some good info, and at the bottom you can download there medium trust config file.
I am getting ready to write some front end code to allow guess bloggers for my site also.