Not entirely sure what alloy "x-fusion quaternary aluminum" actually is, but I did see that some other alu fuji bikes from that era listed as 7005 series which can be TIG welded without needling full heat treatment like is required for 6061. Fuji also had frames of "x-fusion quaternary aluminum" mated together with carbon rear triangle tubes which I would think would be more difficult to fabricate if the aluminum used had required full T6 heat treatment. (Heat treating the combine aluminum + carbon together would likely not work).
If you do try a TIG repair, it might be advisable to fabricate a solid aluminum internal plug, spread the break enough to tightly insert the plug into the dropout end of the broken tube, and then fit it back together with the plug protruding and spanning the break in the tube. Then, TIG weld the plug and broken tube ends back together with the internal plug providing further support for this weak area? Weight gain from aluminum plug would be negligible but it could increase chances of a successful repair.