According to Reddit [1], you can use the existing rg.el package, and just point it to the rga binary instead of the rg binary, and it is supposed to just work.
Huh, thanks yeah that does switch the binary to rga, but with rga you need to specify a wildcard operator for the path parameter in order to search PDFs, otherwise it only searches plaintext files, and I'm not sure how to make rg.el's RG function add that... must be a variable but not finding it.