It applies to both tests and benchmarks. The CL to take care of this (https://go-review.googlesource.com/#/c/19122/) is the last step of the subtest/subbenchmark implementation and has taken a bit of time to review. You will be able to select subtests and subbenchmarks as you suggested once this CL is in.
(the hanging was a silly thing that is fixed in tip, btw).
@bradfitz: that was, in fact, one of my main motivations to start doing this in the first place. Run enables a bunch of other functionality that people have asked for, but I'm looking forward to using this one myself. :)