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So I'm risking it riding some 23s I had spare on my 19mm inner training wheelset
Only if they're abnormally narrow 23s. Since they were probably designed for 15mm rims, I'd expect 23s to come up to at least 25mm on 19mm rims. 19/25=0.76, which is barely into experimental territory.
I've got 23C Contis on 19mm Rigida Sputniks on the RollerFort, they are well over 26mm wide on those rims.
You're in experimental, but probably not yet dangerous, territory when the inner rim width exceeds about 75% of the true (not nominal) tyre width. You can get away with more at the back than at the front, because the front leans more.