I'm playing this alongside Yttr, and I want to add Gadolinite to the drops table for the Quarry (likely in the steel pipe pool). How would I go about doing this? Is it possible as of right now?

Yeah, it's possible. As easy as modifying any other recipe.
Recipe found in the 1.16 branch.
From there you'd just need to add another output object for gadolinite, like this snippet here:

    {
      "item": "yttr:gadolinite",
      "amount": 8,
      "probability": 0.015
    }

And then you'd just load your custom recipe in a datapack.

Exactly! Thanks @Birgliss.

Out of curiosity: It looks like it's possible to add quarry recipes for other kinds of item pipes, or possibly any item. Is this supported at all?

Yes, for example when AE2 is installed we have a recipe to turn ME cables into certus quartz ore.