First of all, congratulations for this nice plugin. I'm using it to manage the projects of my team and we found it very useful.
We are making some modifications (actually I'm are testing them already) in order to allow more than one user to work on the same ticket. We do a lot of pair programming so it's quite common to have two members of our team spending time on the same tasks.
What we are doing is using a ticket custom field (worklogusers) which contains a comma separated list of users which are allowed to work on a ticket. Users listed here can start and stop work on the ticket. This option can be activated/deactivated using a new worklog option called multipleusers.
I don't have all the time I would like to code this, but I hope next week I can submit a patch with the new functionality. We have used revision r2571 as starting point.