The service manager I've gotten to know pretty well locally told me the recall should be a fairly quick fix. ~1hour. As far as the tune, it shouldn't be an issue especially if you mention it beforehand, but it can't hurt to err on the side of caution if you choose to not say anything. Better to spend 15 minutes to de-tune / retune your car, than to take the odd chance some new tech gets in there and bricks your setup because they didn't know.
Here's my thought on it. You're going in to have an airbag recall performed which only involves removing the old airbag and installing a new one... what part of replacing the airbag involves hooking up the Ford software to check if the engine has been tuned?
__________________ Sold - Black 2000 Mustang GT, gone but definitely not forgotten
- Silver 2007 Corvette Current - DHG 2001 Bullitt
Are you concerned about them knowing you've tuned the car or not honoring the recall? Once you've tuned the PCM had a flash count. There's no going back after you've tuned they will always know. Nevertheless a tune does not void your warranty. It does void any damage the tune may have caused.