How to enable our own library to run in the WPF browser application.

You use the same mechanism that applies to all .NET components. If you mark an assembly
with the AllowPartiallyTrustedCallers attribute and install it into the Global Assembly
Cache (which can only be done if the user trusts your code and decides to run it),any of the
assembly’s public APIs can be called by any XBAP

WPF unleashed, page 230