How are non-carbon stereogenic centers named (S/R)?
but I can't find any canonical reference as to that.


Look harder ;-)

It seems to be that the lone pair has lowest priority.

OK, I can't find any authoritative reference to that (I don't have access to the original paper), but I've got plenty of others:

Alright, IUPAC says:


  Except for hydrogen, ligancy, if not already four, is made up to
  four by adding "phantom atoms" which have atomic number zero
  (0) and are thus always last in order of preference.
  
  This has
  various uses but perhaps the most interesting is where nitrogen
  occurs in a rigid skeleton, as for example in a-isosparteine (14);
  here the phantom atom can be placed where the nitrogen lone pair
  of electrons is; then N-i appears as shown alongside the formula,
  and the chiraiity (R) is the consequence; the same applies to N-i6.
  Phantom atoms are similarly used when assigning chirality symbols to chiral sulfoxides (see example to Rule E—4.9).


When ligancy is less than four, we obviously have some lone pairs hanging around (except in Hydrogen--which has been excluded, and some other cases like carbocations--which are planar anyway)

So the lone pair becomes a "phantom atom" and has 0 priority.

Now I wonder what happens to these rules if you have two lone pairs, one of them muonic ;-)

Check the first revision of this post for my original, non-authoritative references