I've asked the guy to upload the GLM Importer code, so perhaps Archangel MIGHT be able to make an importer for XSI? It doesn't import bones or weights though, so it would only be meshes.
That's better than nothing. Though chances of Archangel making a Softimage plugin is slim at best since he only uses Softimage when he has to, he mainly uses 3ds Max.
However, a guy has recently made a GLM importer for newer versions of Max.