License filters! Exellent idea. However, I think it is a bit anal asking for all golds. Shows above average understanding of technique, game physics, and overall driving abilitiy to just obtain an S-License, IMO. It also shows motivation in that someone is willing enough to put more into the game in order to get more out of it.
However, those who have licenses are not always the most compentent drivers out there. It really just takes experience and practice to run clean laps. Each track and each car is different, so it just takes sheer game time to be skilled enough to race cleanly but effectively. Licenses are educational but anyone can devote a weekend to smash through them and still end up as sloppy as someone who doesn't have all the licenses...
The way I see it, there just isn't a good way to filter in GT5. But I'm sure the best results are a stack of limitations, a book of penalities, and a lobby title that clearly states what you expect out of those who join. This usually shows those baddies that your lobby is serious business, and then I'm sure you'd benefit from a similar result as if you were to throw a Gold S-License filter in there. You can't get away from baddies, they'll always be there at any level of gaming.