Does anyone know anything about the Kopell marimba concerto?

neat piece. fairly difficult.

From a performance standpoint, I haven't heard it, but I did read through it recently, and it's now on my list of pieces to learn. I really liked it, but it seems to present some pretty major technical challenges.
From a research standpoint, there is a dissertation (or maybe it's a thesis?) about the Koppel Concerto. If you are affiliated with a university that subscribes to UMI Proquest Digital Dissertations (I imagine most university libraries do), you can download a copy for free at this website:
wwwlib.umi.com/dissertations/gateway
(the author is Robert Brudvig).
You might also be able to get it through interlibrary loan if you have a university affiliation.
He gives performance suggestions, stickings, etc. in this paper, so it would definitely be worth looking at if you decide to play this piece.