"upstream HEAD" or "either of these branches" revision specifier

[shish]

I fairly regularly want to rebase my in-development work on top of upstream

In some repositories, I do this with sl rebase -s 'draft()' -d master, in other repositories, I do this with sl rebase -s 'draft()' -d main. I switch between several projects with different conventions, so there's a good chance that whichever one I have in my shell history / muscle memory is the wrong one.

Is there some way I could more generically specify "rebase draft commits on top of upstream HEAD"? Or maybe "rebase on top of (main or master, whichever one exists; or use the first one if both exist)"

(I've looked though the revset docs and couldn't find anything, but maybe I just missed it?)