I'm not so sure that everything we do NEEDS to be conservation framed. I know this has been re-hashed a zillion times on the G, but sometimes it just isn't practical. :bdh:
This is a guitar pick - get creative, make invisible mounts if you want, use a visible mounting method, but not everything needs to be encapsulated in mylar.
If the guitar picks are pretty standard plastic ones, which I would imagine they are, then if you really just wanted to stick them down to a piece of matboard - why not just use a dab of Elmer's or some silicone? Either of those would come off of a plastic guitar pick with minimal effort and no damage.
If the guitar strings seem like a good idea - put a dab of silicone on the back of the pick and the guitar string that runs behind the pick to help hold it in place.
Whatever you do - Never use superglue for anything! I worked for someone who used to glue hockey pucks to mats with superglue - I told her there had to be a better way, but she told me I had no idea what I was talking about and she had been framing for 20 years already!
It was fine, right up until one of the other employees told her to glue down the WRONG side of a hockey puck. When the customer came in to pick up the piece, he went ballistic when he found out that the puck was not only super-glued down with the wrong side showing, but also that the puck was irreparably damaged!
Apparently it was worth some real money! Our shop ended up having to buy the guy another three pucks to replace the damaged one since the there wasn't actually a replacement puck to be found. DON'T SUPERGLUE!