We had a fairly large wedding in newport last summer (July 4th weekend). (About 150 ppl). We reserved the entire pearls of newport (but had to pay the entirety in cash in advance and then collect from our friends which obviously is far fr ideal) the entire newport blues inn (which was great- they held the block open for a time but didn't require much else to reserve), and then had a block at the Viking. Also check out the Pelham hotel (it's really nice; but someone else snatched it up before we could), and the newport harbor hotel and marina. Can't help you with brunch, we had everythig catered, but maybe try The Mooring?
I am getting married this upcoming July and with 280 invitees, we had the same issue. What I ended up doing is reserving 6 hotels in Middletown that are very very close to each other (some share parking lots). Each hotel gave me about 20 rooms. (Holiday Inn, Howard Johnson, Residence Inn, Atlantic Beach Club, and Hampton Inn.
We did the Howard Johnson, Marriot, and Hampton Inn 'trifecta' as I called it. They allowed about 10 rooms at a time I believe but after speaking with the person in charge I was able to get it increased, and once the blocks filled up they would add more rooms. So I just urged guests to book early! Plus across the street is Brewskis which is the best dive bar, it was awesome for getting everyone together
The Holiday Inn in South Kingstown offers group rates /wedding blocks. We don't require a 2 night minimum stay and the rates are almost half of what Newport hotels charge. We're on the way to Newport for those coming in from CT, NY or NJ. Big outdoor pool, lounge, Italian restaurant. Close to Narragansett beaches too. Ask for the Sales Manager, Marie Fonseca. 401-789-1051. We're about 9 miles from downtown Newport, 15-20 minute drive but our side of the bridge is very pretty and the restaurant and beaches are great!