I actually used to live in Sylvan, so I know a few tricks. Sylvan IS busy. Very true ... but if everyone is willing to get there before noon, parking shouldn't be much of a problem. The bigger problem is access to firepits - there aren't any anymore.
Gull Lake (East of Bentley) is 1/2hr north of Sylvan, and would also make a great spot for a meet. HUGE parking lot right off the lake, and I am pretty sure there are still firepits in the day-use areas.
There isn't much out by Rocky or further west, lots of small campgrounds, but not large enough for a meet.
I would try to stick with Sylvan or Gull Lake.
-=D