Great idea.
The best place to do that currently is to send a message using the
(soon-to-come) wiki features. This will create the expedition page
using the default template (unless it exists), and add your message
("I will be there when at the first light of the day, yadda") to the
expedition report section.
Actually the best place to do that is the participant list section in
the same template. Basically, we would have to extend the "send a
message" dialog with a checkbox (or a default behaviour) that adds
your name to the participant list when you send a message.
To query the current participant list, you might just the existing
"show expedition page" button. It has to be fetched anyway to query
the participant list.