official d20 Glorantha unlikely
I believe there was a previous brief discussion of this. My understanding is that Greg Stafford is fairly hostile to the idea. Given that he publishes his own system for the setting (which her created) and how radically it differs from both RQ and d20, I doubt that's a view that will change. Provided you are rspectful of his creation etc however, I can't see him having any objections to a fan conversion.
My instinct is that a lot of nuts and bolts work would need doing to the d20 system: the base homgenised technology and cultural assumptions would have to be scrapped, involving alot of work on the weapons, armour and such. Magic would need a lot of work as well.
If you can get hold of it, the Glorantha book for Hero Wars is a good overview of the setting that (for all it's a Hero Wars supplement) is pretty system neutral IIRC.
Best of luck!