Hi there, thanks for the suggestion!  We've actually made these in the past for customers on a custom basis, and we're working on a whole new series of adapters that will let people power anything from pretty much anything else.  So, think PF, PU, Control+, 9V LEGO...pretty much anything.  Stay tuned-- we're testing prototypes now.

Also, just one more comment on the 9V battery idea.  Many people think 9V batteries are a great solution, which they are-- for some things.  Amp for Amp, however, AA batteries actually have more *power* (not voltage) than 9V batteries.  They are actually pretty disappointing when used with components like motors.  Much better are similarly-sized LiPo or LiFe (even better) battery packs-- these have a much higher energy density, and when using LiFe batteries, they don't have the same fire/shipping risks of LiPo batteries.  We're working with those also in our designs, to give people the widest array of options to power their stuff!

(oh, also forgot to mention USB-to-PF) 😉