Best is to file a bug with FPC. The example program of comingnine is already fine for that. Tomas has been working on this problem (fexpand/expandfilename) since FPC 1.x times, so getting him on board is important. It's quite complex because UNC and Amiga/Novel volume notations.
While there is only expandfilename routine, it is parameterized with ifdefs, there is a small chance that the outcome per OS could be different. So add test results for at least windows and *nix to the report