Gosh, many of us have asked about this before, and it's never been brought up by the devs. There is a wand from the Flames gearsets and another from Adders that I love but I can't use in glamour for the exact same reason. The game just doesn't register that it's a main hand weapon.

I understand why they opted to drop the off hand stuff for mages for balancing reasons (easier to ensure your stats stay were they need to be if you have a single weapon), but they just refuse to go back to make the wands BLM and WHM had glamourable over the high level gear.

It's so annoying!