Can I run a shimano medium cage 9 speed rear mech on a shimano road 10 speed block (or a dynasys 10 speed mtb block)? (with a 10 speed chain) Would rapid rise or shadow mechs cause more issues?

I've got a feeling it should work but would rather know for sure rather than spend money to find it doesn't

dynasis is a different pull ratio to 9spd mtb so needs specific mech / shifter to work on a 10spd mtb block. 9spd short cage works on road 9spd but jockey wheels may need a swap for 10spd chain and the angle of the mech cage won't suit a roadblock. it'll work, but not that well.

better to use a 10spd flatbar shifter and road mech/block for ghetto 10spd.

What shifters are you using? 10 speed road or MTB?

Perhaps I should ask a slightly different question. Is there any option that gives me a 30-something bottom gear I can use with 10speed shimano drop levers?

Yes, an MTB 10 speed cassette and a 9 speed MTB mech.

I thought that didn't work due to the cable pull differences though?

excellent! I'll put in an order

Yep, works fine. You can't use a 10 speed MTB rear mech though as the cable pull for that is different.

Aaaah, I missed the bit about it being drop bar levers, I assumed we were talking about 10 speed MTB shifters. I'll go back to my cave.

More coffee Nick?

Got anything stronger!?