I prefer a 'we'lll deal with it when the time comes approach' The bulk of the meet has nothing to do with pubs anyway...
I think its too unpredictable to try and anticipate the pub situation anyway.
For the sake of comparison, the second time we had the meet, when it was my birthday there was no problems, we had one person that was 17 and it was a non issue. The first time we did the meet however, we had one person who wasnt allowed in, in which case we went to a different more seedy place outside of the touristy lest. square area and we had no issues.
Anyway, im not really condoning that people who know they cant get into pubs, try to agressively attempt to. I sure as hell will not be babysitting nor should anyone have to do babysit you. I'm on vacation as far as im concerned and so is everyone else. It shouldnt be our problem that in attempt to be cool you are 'that guy who cant hold his liquor'. There will be plenty of time to hang out, if you skip the whole pub part you really didnt miss a TON of activity.
If the group isnt too rowdy after a few drinks, we may walk back to my flat to get out of the crowded pub scene and continue conversating, maybe ill even offer some of the floor to anyone who needed a place to crash..... I'll evaluate that when the time comes though.