Once you have a journal set up, you can add this command to your game, removing it just prior to release:
<command name="bug">
<pattern>bug</pattern>
<script>
msg (Template("WriteInJournal"))
s = GetInput()
UserAddToJournal ("BUG:" + s)
msg (s)
</script>
</command>
Anyone bete-testing your game can type "bug" at the command line, and then type a comment into the journal (with "BUG" prepended). Your tester can then copy-and-paste the entire game session, and sent it too you. You then search through for "bug" and you can see his comment, and exactly what he was doing to provoke it.
What you could also do, if you are familiar with libraries, is create a specific debugging library, with various cheat commands (say to teleport you quickly to a certain place), plus the above command. When it comes to shipping your game, swap the debugging library for an empty one with the same name (and then run your walk through to check nothing was broken).