I think i'd want a bit more than "POSSIBLY build a spec home in the future", but if you have a customer who builds spec homes for a living and they buy a tract of land to (definitely) build one later (not with your loan), i wouldn't have a hard time exempting the land only purchase loan from Reg. Z as business purpose.
I'm assuming this loan is to an individual....if to a non-natural person (LLC, etc.) then it's exempt anyway.
I'm fixin' to fix that.