Ok, I had an idea.
It is possible to build .dll files with Lazarus, compiled for arm7?
That would solve my problem, I would just build a dll with all my code and then bootstrap it from a c++ Windows Phone 8 app.
If not possible, how simple/complex would be to add this feature to fpc?
From what I know, fpc already can build Windows dlls, I expect the file format would be the same, with the only difference being the generated code, that would be arm7 instead of x86