Sorry for the late answer, end of the year time ye know.
BRLs are somewhat like inserters as far as i understand, i.e. they are what does the transfer in the first place, so it's relatively easy to add filters. Stockpiles are just large chests and the engine does not support filtering for standard inventories. So even if i wanted to i wouldn't know where to start. And it would further increase how many different "stockpile" items you have to handle (i didn't want 12 items which is why logistic modes work the way they do).
And besides what would be the benefit? They'd still be the same size. They're also already far more realistic than chests merly by requiring actual space instead of magically compressing everything into 1m³ :p.
TL;DR: Nope, sorry. Can't do. Hope you enjoy the mod anyway.