I appreciate the suggestion - any tips (or resource suggestions) for increasing the chances of successful rearing (of this and other collected young insects)?

I would put it in a small ziploc-type bag with a folded-up, slightly damp paper towel. That's what I did with this sedge-mining Elachista that I successfully reared recently. After it pupates, the pupa should be transferred to a vial or small jar, because a moth emerging in a plastic bag will rub off a lot of its scales.

Should I keep the moist paper towel with the pupa in the closed vial? It dropped out of the leaf and silked itself to the paper towel, but I could let it dry out, probably. Thanks!

You can definitely let the paper towel dry out some, to keep it from getting moldy. Best not to let it dry out completely, because you want a little humidity in there to keep the pupa happy, but you can always add a drop of water later if it gets too dry. It looks like the larva hasn't quite pupated yet, and I'm not quite sure from this photo whether it's the moth or a parasitoid, but I guess it must be the moth since I don't see a dead larva in the leaf. If you still have the leaf, it's a good idea to press it so it can be examined later.