I mean It works for me after my changes: https://github.com/alexeyvo/mongo-delphi-driver/commits/d5
I made 2 versions of Seek(for longint and int64) as it defined in TStream of D2007 and D XE5 but as I understand it didn't work on modified VCL but I can't test it. I just saw that D2007 and D XE5 have 2 overload versions of this methods and thought that it's better to add similar for D5 instead a lot of {$IFDef} conditions.
So I think I need modified VCL anyway to test everything on both.