Nailed it, thanks so much! The Asana-Enable = new_memberships did the trick. And follow-up on my other comment. Those standard_access_levels are (unsurprisingly) documented correctly, so comment_only instead of CONTRIBUTOR, per the error. The custom header fixed that. 
Also did a bit of tinkering re: potential mapping of standard_access_levels to the membership’s notifications settings—no dice, even with the most restrictive comment_only. Based on the docs for this endpoint, I just assume that’s not a thing…yet.
@sasha_f that ability to set notifications via these endpoints (or something similar/related) would, IMHO and for our use-case, make this feature complete…for what it’s worth. 
Thanks again, to you and the team, for all your work + communication on this!