So I thought about how to attach a diagonal or camera, neither of which will come to focus on the stock OTA. The solution was to cut the OTA at the inner draw tube collar. That was straight-forward, and I happened to have an empty barlow tube floating around, which just happened to fit quite nicely, to serve as the new 1.25" interface. The inner 1.25" tube was cut off the dud barlow as well to minimise any vignetting. Focusing is currently by adding/removing extension tubes and thumb-screwing
Getting something attached to address this will be the next task.
In the meantime, it is possible to take photos through it.