Pro develop to Free

Is it possible to create a DLL on Unity Pro that uses some of the more esoteric Pro features (Graphics.DrawMesh and C++ DLL interop) that can be used in Unity Free, as long as Free doesn’t recompile the code?

No, if you compile a dll / assembly it’s just a plain .NET / Mono assemlby. You can compile such a dll with any .NET compiler. Your whole build is marked as indy or pro build so the Unity player knows what you are allowed to do and what not. Interop might work since mixed-mode assemblies seems to work in the indy version, but haven’t tested it yet.