I've asked Michael if I can explain my logging-to-file proposal at the meeting. Trying to gather support for it.
On the down side, if he says yes, I will have to get out of bed in time for it!
Martin, we (the OU) did that in our Moodle 1.9 site.
In Moodle 2.x we have achieved a similar speed-up with a sneaky trick. mdl_log is a view that aggregates a small table where write are redirected, and a bit table that is basically read-only. The advantage of doing it this way is that it is transparent to Moodle, so no core code changes.
Even so, a very large mdl_log table still starts to cause performance problems. Talk to anyone who hosts large Moodle sites. The only option is to regularly throw away most of the log data (or copy it to somewhere else if you really want to keep it) but then it is really no longer joinable, and anyway we want to keep this data for analytics and research.