Also, can anyone provide a summary of all the DB-related libraries for
  Guile?  I believe there's guile-pg, and guile-sqlite, but (i) there
  could be others I've forgotten, and (ii) it would be great if someone
  could summarize how they differ / overlap / compare with each other.

I am familiar with guile-pg.  It used to be on sourceforge, and was
unmaintained, and then ttn took it over and has done a lot of work on
it.  Last I checked guile-pg only supported guile through 1.6 due to it
using hte load-module-from-.so feature removed in 1.8, but ttn has said
that he'd take patches to make guile-pg work with 1.8+.

There is ttn's guile-gdbm: http://www.gnuvola.org/software/guile-gdbm/

There is no guile-dbi package in pkgsrc.

I dimly remember a pg-guile which was BSD licensed but have not heard
anything about it in years.