I ended up figuring it out, in a roundabout way. I'll try and remember to post the solution later for posterity, but basically I got it close to what I needed using bean-query, and then piped the output of that to awk where I summed the column.
In my case, I needed something that could be scripted. And this did the job.
I'm new to awk but there are tons of cool things you can do with it, especially with structured data.
I would still like an answer on whether this is supposed to be supported or not, because I have searched and searched but maybe I missed it or I dunno what, but I sure as hell haven't been able to figure it out yet (strictly within Beancount tools).
TRS-80