As in: have, say, another file which contains user names, and have
   utmp/wtmp point to entries in the second file?

no.  in the SVR4 model, there is the utmp file and the utmpx file.
the "system" keps them in sync if you use the correct api.  the
utmpx format is similar to the utmp format, except with larger
values for various elements.
   
   It seems that that's slow, unnecessarily complicated, and unless done
   carefully can be prone to not keeping accurate track of the
   information.

that's why we need to do it right (unlike SVR4).
   
   > you could also "break" binary compatibility but retain the ability
   > to read the old format with updated tools that do both.
   
   That's relatively difficult, since the files and/or records in
   question have no magic number by which you can determine their
   versions.  Sure, you could use a heuristic, but it's quite likely that
   it wouldn't always work.

i don't think it's unlikely to always work.  the utmp has a very
defined format.  there are almost always nuls in the `right' places
where you would expect them (at the end of the char array fields).

you could use some kinda of magic number for the new format that
was extremely unlikely to be repeated by the old format.

.mrg.