TBH anything held in the straight arm position is likely to have minimal positive carryover. Obviously very high level gymnastic positions are the exception, but in the exercises mentioned above the main loading is still being put through your skeletal system. Your muscles don't really have to do a heck of a lot.
Well he could perform handstand push ups, which are pretty decent at activating the tri's, but yeah the standard handstand is more of a balance/coordination exercise.