There's no way to enforce anything in the LFG tool. People queue as tanks and healers to jump the DPS queue all the time, people would queue for any sort of "No Rush" thing as well and then take off to wipe the final boss before the rest of the group has taken care of the first.

Unfortunately, all you can do is either make your own group from friends/guildmates/advertising looking for a slow group in zone, or take your time and go through the dungeon at your own pace regardless of how fast anyone else is going. Whenever I get into a pug I'll generally wait for those who are lagging behind for whatever reason. Also, if you want to take a dungeon slow or not skip certain bosses, you can always mention it in group, and if they ignore you, do your best on your own in the case of handling bosses you don't want to skip that everyone else has.

I personally hate that about FG btw. Most everyone will jump at the waterfall to skip the Druegh and Crab bosses without even asking, when it only takes a handful of minutes to clear those bosses too and get their drops. If you can't spare an extra five minutes to clear a dungeon, maybe you should wait until you have more time to play to be queueing.