Yeah, I briefly talked to Lennart about how to do this.  Luckily
the journal format is backwards compatible (and semi-forwards
compatible too), and there's a sane library for reading it:

http://www.freedesktop.org/software/systemd/man/sd_journal_open_files.html

One option is to add this API to the libguestfs API (rather
like how hivex is done now).

Another is to have some guestfish command to download the journal
files into a temporary directory and run journalctl -D on it,
although this would be rather slow.

Simple matter of programming :-)

Some support has been added to the API now:

https://github.com/libguestfs/libguestfs/commit/5cb7f294f6a5a8b4a68e940c30ca9af0fe0741d5
(in libguestfs >= 1.23.11)

It would be nice to enhance guestfish further to add a journalctl-like
command for browsing the logs, so I'll leave this bug open for now.