I'm sure I saw a discussion about Flexcel on fpc but can't find it. AFAIR there was some limitation of the freepascal compiler - is this still the case?
We could workaround this one by changing class.Method<T> to class<T>.Method, but then, XE is buggy and won't work with class<T>.Method. XE2 is fine, but many people is still in XE. This are the joys of supporting many different platforms :(
Note that this might not be the only showstopper. I for example think they also don't support anonymous methods yet.
We are still waiting for those features to come. Some years ago, it was completely unthinkable to compile FlexCel 6 in fpc, even things like dots in filenames wouldn't work. Then they added a lot of stuff and we got optimist we could add support soon, but the missing features seem to be taking forever. If we had more resources I would look into manually workarounding those missing parts, but right now we can't do it.