I knew about the AQHA/APHA dual registry, but from what I understood, the horses eligible after this new rule to be registered were simply the AQHA horses who weren't allowed into AQHA registery and would have otherwise been registered AQHA and not APHA if the quarter horse association allowed excessive white and double diluted horses. Sorry for speaking wrongly.
I also, would not buy a horse relying on the fact that you may be able to get AQHA papers on her. I would make sure I was completely happy with the purchase of this filly with SPB APHA papers and consider the AQHA papers as a bonus if you are able to obtain them later.
Did you ever buy the two fillies you were looking at before?