thinCore.inc has all the current interfaces available for thinBasic.
Usually there are not much troubles when dealing with function interfaces dealing with numeric parameters or numeric return data.
Problems can be when dealing with strings because, as you know, there are many different ways to handle them. If FreePascal manage BSTR strings I suppose there will be few problems too.
Also, if you need, I can create new interfaces. We just need to talk about what and how.