Lovely. After a quick look through that discussion, it seems that the "right" thing to do is install into whatever
$ pkg-config systemd --variable=systemdsystemunitdir gives, and one day that will swizzle but there's no need to worry about it. And in the meantime, no need to pollute pre/post scripts with reloads. Barring objections from people more knowledgeable than me, that's what I'll do to close this bug.
