Hi Craig,
Sounds like the "security update" is that they switched to enforcing Medium Trust from the machine.config on the server.
More and more hosting companies are doing that but its kind of uncool of them to do it without warning ahead.
mojoPortal can be configured to run in Medium Trust
http://www.mojoportal.com/mediumtrust.aspx
but you may have to sacrifice some functionality. For example if they aren't willing to install NeatUpload in the GAC you won't be able to use NeatUpload. It can be commented out in web.config to disable it. There are other things that need to be configured differently in Web.config too, you should see comments int here about Medium trust configuration.
Also if you are using a db other than MS SQL, you may not be able to get it to work unless they install the ADO.NET driver in the GAC, like for MySQL it would be the MySQL Connector for .NET that would need to be in the GAC.
Hope it helps,
Joe