yes i know for md, and you can change (not by changing a registry of course) BG priority easily .
TF4 water stage uses extensively this feature .
Some snes games do the same .
I did some tests with charle's doc, and i found VPC not very useful(in common use), because the only serviceable sprites priority is SPR2 in front of BG1 & BG2 ..
You have only 4 values, two are default VPC parameters, one is cool but usable in some restricted case, and the last for SPR2 in front of BG1 & BG2, the only useful .
It seems impossible to do SPR1(with high priority) in front of BG2, but behind BG1 in mid frame ..