I've seen a lot of literary magazines and journals that take submissions for novel excerpts. Are those typically published novels only, or is it okay to send in an excerpt from a finished, but currently being queried novel?Generally excerpts are from UNpubbed novels. Check the submission guidelines for the magazines in question about what they take. It varies widely.
Also, if the excerpt was accepted for publication (likely with rights reverting to the author on pub), could that then be added to the query bio?
If the excerpt is published, even if the rights don't revert to you, you're fine. In fact, most short story
collections include previously published stories.
How to query for this: write "excerpts from this novel have been accepted for publication by The Good Taste Review" or "excerpts from this novel have appeared in The Better Taste Review and The Carkoon Review."
And yes, you really should include those with your publishing credits in a query.
When your book is sold, you'll include "an excerpt from this novel in a different form appeared in The Good Taste Review" on the copyright page.
You'll also make sure your publishing contract acknowledges that excerpts have been previously published.
Your agent will know what to do.
Having excerpts already published is something I view as a plus. It means someone else looked at your writing and found it publishable. That's almost always a good thing to see in a query.