(rdnzl:invoke "System.Reflection.Assembly" "LoadFrom" "c:\Documents and Settings\user\Desktop\Foo.dll") should give you the assembly.
A thing that might be of interest here is that assemblies loaded in a LoadFrom context don't work exactly like assemblies loaded with Load/LoadWithPartialName and so forth; for example de-serialization always uses Load so that won't work; and there are other diverse issues. See http://blogs.msdn.com/suzcook/archive/2003/05/29/57143.aspx .
A fix that works is subscribing to the AppDomain.AssemblyResolve event, which is triggered when Load/LoadWithPartialName fails; you can add your own code there to find the assembly any way you need to. I haven't been able to actually do this in lisp (ACL7) via RDNZL though, as somehow I get a crash when the new assembly is returned to the AppDomain triggering the event. But subscribing to the event in C# code used by RDNZL works fine; I've attached an example class that can do this.
With this class loaded (also with LoadFrom) using Load and LoadWithPartial will work with any other paths you specify.
I think one is probably not supposed to do this either, but it does work and people do use it.
(I've also attached the non-working lisp that should have handled the event in case anyone is interested.)
Iver Odin Kvello iok@selvaag.no