Don't think having a sub forum will help increase the numbers, as this post is in the General LFS Discussion forum, so its probably getting more exposure here.
Maybe you could aim the event towards new comers / people with limited league experience. Lets be honest the UF1 is a easy car to drive and doesn't take much concentration to keep on the track for 120min. Also, why dont you just even the number of racers out over two servers? 16 on server 1, 16 on server 2 etc ??. Doesn't really matter, id prefer the server not to be too full.