Probably a dumb question, but here goes:
I just acquired a nice Tiffen filter set for my GL1
(58mm) / is it possible, by way of a step-down ring,
to use these on my XL1 as well? - or is the 58-72mm
too great of a size drop?

You might get away with it if you shot in telephoto, stop your lens way down (F/16 or more). But that is too limited to be practical. You can go the other way though, 58mm to 72mm via a step-up ring. that will work with everything except extreme wide angle.

I guess I'm still a little dense on this -

are you saying use a step UP ring rather than a
step DOWN ring?

and the operation would be:
XL1>step up ring>58mm filter?

OR am I missing your point completely?

Jeff means you can use a 72mm filter on your GL1 via a 58mm to 72mm step up ring. To use your 58mm filter on the 72mm threaded XL1, you'd need a 72mm to 58mm step down ring.

If it was me, I wouldn't bother using the 58mm filter on the XL1, unless like Jeff says, you plan on shooting only telephoto stuff.