Ham sandwich navigation and radar failure

Question for the enroute controllers. You have a spamcan or two, not filed
/G, cleared direct to some fix. You don't care whether they are navigating
with pilotage, a VFR GPS, or a ham sandwich, so long as they are on radar
and don't go too grotesquely out of the way.

Now the radar goes kaplooey, or whatever noise radar makes when it decides
not to be radar any more.

How do you get these guys into the no-radar rules - point them at the
nearest navaid? Tell them to join the nearest airway? How do you maintain
separation? Does this possibility make you nervous while the radar is
working? I assume this is in the controller's handbook, but forgive my
laziness. Just curious.

-- David Brooks

For the purists, s/radar/RADAR/g. For the pedants, 1,$s/radar/RADAR/g