I'm under the impression that one of the things really holding this feature back is the fact that a chunk of the quest gear you get given actually comes from side quests. MSQ doesn't hook you up with a complete set, so setting a minimum that isn't an inconvenience becomes troublesome.

Don't get me wrong, it'd be pretty easy to just pick a decently average number and roll with it. But, the fact that they haven't suggests, to me, that they don't really want to set it to anything that might cause a player to flounder around trying to scrounge up the gear.

One solution would be to remove all the gear rewards and just condense it all down into a singular coffer of some appropriate ilvl set. Something tells me they don't really want to remove the gear rewards from the side quests, as it serves as some added incentive to learn about the area you're in.

Another option, which would do little for ARR dungeons, would be to at least set the minimum to whatever the previous minimum was. You need an ilvl of 90 to do Steps of Faith prior to HW, so everything from Dusk Vigil onwards should be set to ilvl90. If you really wanted to be clever, you could take just the gear rewarded from MSQ and calculate the ilvl you could achieve from that, with the ilvl 90 gear for everything the MSQ doesn't reward.
The minimums would be incredibly low, quite likely not truly solving much, but they would still be enforcing a minimum, while still satisfying the dev desire to just let the player roll through without stopping and shopping.