I asked Hash Cows about it I'll let you know if they respond favorably. They just added zeit and some other coin so now that's 3 they allow payout in, can't see why adding HBN would be a problem.
For me it may be the volume, if you want to buy a "lot" of HBN "right now", there is not that much offer to do so...
then the laws of supply and demand kick in. If you need to pay higher prices to fill orders let it be so. It just means that HBN is actually much more valuable then it is now. Limiting mulitpools to coins like MINT, ECC or ZEIT doesn't make sense because percentage-wise, they can move just as much if not more than HBN.