Is a Vittoria Randonneur 28- or 32-622 on a hook-less 23 mm rim feasible?
Hookless is getting more commonly seen and will probably become pretty normal. More analysis here: http://www.bikeradar.com/us/mtb/gear/article/trail-tech-off-the-hook-40749/ . Basically, the bead lock area of modern tubeless compatible rims diminishes the need for it. For this kind of rim (very short sidewalls, center mounting groove, bead lock), lack of hooks shouldn't be a factor in figuring out what width to run.

What will actually get you in trouble, or keep you out of it, when determining how narrow you can go is how much tire you have overhanging the outer sides of the rim, which protects the rim from dings and scrapes. A reasonable minimum is 1-2mm per side, and never a tire whose inflated width is narrower than the rim. (There are some trends in the racing world to break this rule, but rims suffer as a result.) Internal width is a relevant consideration, especially in gaging how wide you can or should go, but it's not the end-all factor that it's sometimes made out as these days.