If you will have the tire off you are more than halfway there!
I suppose it could be the bearings seeping...
I just had my local dealer put new Dunlop Wide Whitewalls on my 1998 and I notice specs/spots of lubricant on the hub now. I have cleaned them a couple of times but they have come back. Not a lot, mind you, but enough to be noticable by me. If it continues after this cleaning and does not diminish I will be going in to see where the source is myself.
Hopefully when you get in there it will be kinda obvious what the source is for you.