This PC is on battery backup (UPS) already - that didn't stop the corruption though. This is a home PC, and in any case it really shouldn't be necessary to use a UPS just to avoid filesystem/LVM corruption.
Since the rebuild, I have realised that the user of the PC has been turning it off via the power switch accidentally, which perhaps caused the write cache of the disk(s) to get corrupted and is a fairly severe test. Despite several sudden poweroffs due to this, with the new setup there has been no corruption yet. It seems unlikely that the writes would be pending in the disk's write cache for so long that they couldn't be written out while power was still, but the fact is that both ext3 and LVM data structures got corrupted.
It's acknowledged that ext3's lack of journal checksumming can cause corruption when combined with disk write caching (whereas XFS does have such checksums I think). The only question is whether the time between power starting to drop and the power going completely is enough to flush pending writes (possibly reordered), while also not having any RAM contents get corrupted. Betting the data integrity of a remotely administered system on this time window is not something I want to do.