"From a one armed concentrator you can advertise any route (more specific of unique) you want to exit that concentrator."
Spot on.
I completely forgot that you can define networks on a one-armed concentrator without them being local to the MX.
That fits into the design perfectly.
"Can you get a static IP address for each spoke - that might be the easiest option."
I agree this is the cleanest and my preferred option.
However I'm preparing should we not be able to go down that route (which seems to be a distinct possibility)
Thanks for the quick responses!