IMO, a GT1 should only pass on a straight providing they would have enough time to be capable of moving back to the racing line before the braking zone, or when 'flashed' to pass by the GTC driver. This would avoid some of the 'Is he / Isn't he' type scenarios which are bad for both drivers.
As far as I'm concerned, the lapping driver can pass wherever he can make a fair pass stick - in other words, getting decent overlap before turn-in. It isn't going to work otherwise IMO - a GT1 who is as polite as you suggest might have someone behind him who will stick it down the inside anyway while his opponent waits behind you. The only thing that isn't on is to dive down the inside so late that the lapped car has to stay wide to avoid an accident, and this does happen sometimes in my experience.
As a dyed-in-the-wool GTC driver, I see it as part of the skill of mixed class racing to let the GT1s past at the best place for me, and that doesn't always mean the latest time. If I force a GT1 to pass down the inside into a corner then I tighten his line and slow his exit, and this may well compromise mine. An earlier lift might be a better solution, although it will depend on the circumstances (have I got another GTC right behind?).
