Hi...
As far as I know, no hotel will allow outside catering. The average cost per head in a hotel comes around Rs.300 to Rs.400 excluding taxes. But if you add up the cost for arranging a separate hall and outside catering, that will also come close to this amount.
If you are looking for vegetarian, then saravanabhavan halls might be a good option...The cost is also less compared to other veg hotels, starts from Rs.200. The hall is also reasonably good.
It would be a good idea to have someone personally visit the halls/hotels and find out...you will have a lot of options/deals then...
I have mentioned a few places in
Birthday Party in Chennai - A GUIDE
Will add more info to that thread as I get them as I am also on the lookout for a hall for my kid's birthday. You too can add your findings there.
Radhee.