That was interesting. Just prior to sleep I was debating whether to set the alarm for 3 or 4.5 hours (allowing for recovery from yesterday's Zerchers). I opted for 3, but woke naturally a little over 4.5 hours later. I suspect the alarm went off, I stopped it quickly and simply went back to sleep.
This incident was enough to pursade me to try the 3/4.5 hour approach (3 hours by default, 4.5 hours following workout days). Either way, it's a lot less sleep than the 'normal' 8 hours - without any noticeable loss of quality - so I'm happy with that.
Following nap
Tonight the nap was split as I was out during the usual period. I had a chance to take a brief 20 minute nap early, and a 30 minute one later; these seemed to do the job fine. I certainly don't feel any worse for having altered things slightly.
This flexibility bodes well for future alterations (although there aren't any new ones that spring to mind just yet), and means that working late/going out etc shouldn't be a problem.