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.

guile-pg has really nice support for converting postgresql types to and
From scheme types, and you can register custom converters.  So it's much
beyond just being able to run query strings and get string answers.  I
have the impression that it's implementation is tied to pg.

IMHO the right thing would be to use the type mapping part of guile-pg
with a DBI sort of back end to go to various databases.