Any question answered...

Assume it's not possible to bodge anything less than 10speed since it won't be able to move far enough?

The rear dérailleur travel is the same for 8, 9 and 10 speed (they just squeeze more sprockets into the same width by narrowing the chain), so you can use an 8 or 9 speed RD on a 10-speed cassette. In fact, you can probably get away with a 6 or 7 speed RD too, as they had only about 3mm less travel and there was always some wiggle room to allow for dropout geometry variation, hence the limit screws. Potentially, using a lower speed RD makes shifting slower/less precise as the cage plates get progressively further apart as you drop down the speed range, but it only really gets bad if there is a large mismatch.