If I do a biennial flight review (BFR) in a glider, does it also satisfy my BFR requirement for powered flight (ASEL)?
Yes. 61.56(c)(1) calls for "an aircraft for which that pilot is rated." The reg does not require a review for each category/class.
And as further confirmation, AOPA's Pilot's Guide to the Flight Review specifically states:
The satisfactory completion of a flight review allows you to exercise the privilege of pilot in command in all categories and classes of aircraft on your pilot certificate.