Problem is, lots of people have suggestions of nice things they'd like to have Tomato do. Most of these have very small appeal to most users.
I actually think this would have a very large group that this would appeal to. It's a pretty common feature these days on most routers. Every Netgear I've owned has had a feature similar to this.
And there is VERY limited room in the 16MB flash to add much of anything.
I seriously doubt this addition would have much of an impact as far as space. The code in Gargoyle doesn't appear to be that complex.
The point seems moot anyways because according to the updates in the GIT it appears that teddy_b already put this feature in to both branches because he just plain rocks.
I wouldn't mind seeing some URL blocking/filtering as well. If not just for the simple fact that the Access Restriction section is very lonely. :-)