Fwiw, I reckon you're right. I'm intrigued by bullauns (Though annoyingly ain't ever seen one in the real) 'cos they seem like they could be a continuation of the 'big basin' feature that often pops up in association with bona fide rock art in Northumberland and Kilmartin. There are a few RA sites that have what could easily be described (and probably have been by a certain Mr FW) as proto-bullauns. Sometimes the academically approved RA is only metres away. Sometimes it's only inches away:
http://www.themodernantiquarian.com/post/38575/images/football_cairn_e.html
Given the christian propensity for dipping people's heads in water, I reckon there's a fair chance the early christian bullauns could easily be older basins that were appropriated for head dipping duties.
Personally speaking, I am loving the whole bullaun thing. More please :)