Hopefully it'll rattle a little bit less when occupied. After satisfying myself that it's reasonably secure this evening I'll take it out again tomorrow night with at least 24lb 4oz of something in a rucksack strapped into the seat to see how it affects the handling when loaded. I was surprised how little I could feel it affecting my aerodynamics, especially after two days of being surprised how much more aerodynamic I am with all the crap I take to work in a pannier rather than a rucksack. It's a slight pain that the acquisition of the rack and seat coincided almost perfectly with the loss of a couple of mm of maximum rear tyre diameter but I'll just have to go slowly and choose routes based on road quality where possible and try and shift some weight towards the front of the bike. Seeing as people who weigh at least eleven kilogrammes more than me are able to ride bicycles there's no reason (other than normal healthy paranoia) why the frame or wheel should be unable to deal with the weight of me plus the wingpiglet. And the seat. And the locks. And whatever I else I need to carry. My new but not-yet-trued-or-tested wheel will hopefully be strong enough (due to stock issues with almost every rim and hub I was looking at it ended up being 32-spoke) but if it's not then it's easy enough to switch back to this 36-spoke cheapyjob when in child-mode.
Hopefully I'll remember, when carrying something in the seat, not to dismount in the usual leg-swinging manner. I'll maybe stick a wee note on the stem to remind me.