XOXO

No idea what the etiquette is but I do know the etiquette is that you don't invite people to your shower if you don't plan on inviting them to the wedding. So I guess if she is not having a wedding, then etiquette might dictate she not have a shower.

I would say it would be nice to do something very intimate just for her really close friends and family who understand that she is eloping but still want to throw her a shower.

Stef is correct. You should not invite someone to a shower unless they are invited to the wedding.
Does everyone know she is eloping or is it a huge secret. Usually when you elope no one knows but 1 or 2 people. Part of eloping is not doing all of the 'wedding' stuff which includes showers.

Perhaps you can throw her a housewarming party or small wedding reception when she gets back instead.

If you really want to throw her a shower I would keep it to Moms, sisters and very,very close friends only.