How do smaller but good restaurants keep/reheat roasted meat, particularly Prime Rib?
I've no idea about a standard, but a 90-seat restaurant where I used to work served a veal chop intended for 2-3 people by cooking it sous vide to 130F, then finishing on the grill to medium.  We could thereby get a piece of meat that would have to grill or roast for at least 45 minutes to the table in around 15, and the sealed chops could be kept in their bags for 2-3 days.

Treating a whole prime rib roast this way might be a bit difficult, but perhaps you could use smaller slices, sous vide to very rare, and finish quickly in a hot oven (or get all French Laundry and use a freaking blowtorch on the edges).  Or perhaps the other way around - roast, slice, seal, and keep hovering just outside the danger zone until needed, at which time you bring up to temp.  A bit questionable if an inspector catches you, and a bit tough to deal with carryover,  but some enterprising chef might have tried it.

I would venture a guess though that this is why you don't see too many smaller places serving prime rib.  I typically see this featured as a nightly special or only in larger steakhouses, and it often seems to be gone by the time I get there.