American Airlines does this now with "Group 1" boarding passes. It does'nt matter when you check-in, you still get a Group 1 boarding pass if you are Gold, Platinum, or EXP.
I don't see how Southwest could do this when you check in at a Kiosk, there are a limited amount of "A" boarding passes and that's it. If you wanted WN to do this on "the sly", you would have to go to the counter or gate desk which takes time and is usually crowded anyway.
I don't think that other passengers would get upset, if it was a published perk like on AA, it would actually draw MORE business. There may be 2-3 CC holders per flight out of 100 Pax or more would be my guess. That many would hardly be noticed if they checked in with the "A" group or right after pre-board.
Just my .02
CF