I've not done an awful lot of this, but...
- I know that Glenforsa don't mind you camping under the wing. Also it's arguably the most charming airfield in Britain, if not the world.
- I recently stopped overnight and camped at Fishburn, between Durham and Newcastle. Lovely friendly place, gorgeous scenery, very cheap, fuel available. Okay, it's not actually in Scotland, but a long way north within England, so might be a useful staging point en-route North or South.
- Not sure of the current situation, but there was a club caravan on North Connel airfield at Oban that you could make use of.
- There's a friendly SYHA Youth Hostel at Broadford village, which despite what it may look like on a map is NOT next to Broadford airfield (on Skye). Nonetheless, it's a lovely walk - about 4 miles. No idea about camping there, but somehow doubt anybody would mind if you asked.
- If you don't like long walks, there's a hostel (and about 100 B&Bs) a short walk from Plockton. No idea about camping there.
- On Lewis they're happy for you to camp anywhere that doesn't cause a nuisance, so camping on or near Stornoway airfield shouldn't be a problem. That said, I've not tried - the one time I've been on Harris and Lewis, I was backpacking and didn't visit the airfield.
G