Can I use a 9spd 6500 Ultegra cassette/6600 10 spd Ultegra rear mech with a 9spd XT trigger pod?

Assuming that 9spd and 10spd cassettes have the same depth, I can't think of any reason why not. I'm going with a single 40T chainring.

So…

1. Whack on the cassette.
2. Whack on the derailleur and adjust limit screws.
3. Whack on the trigger shifter, cable up and bob's yer uncle?

No hammer required.

Yes that will work fine.

I was wondering this too. Won't the shifter not have enough 'clicks'?

The cassette number-9 needs to match the shifter number-9, the derailleur will move according to the shifter so it will be fine. The derailleur doesn't know it's a 10 speed….

crikey is correct.

As crikey says, all the derailleur does is move the distance the shifter asks it to. So a 9 speed shifter will shift the derailleur enough for a 9 speed cassette.
So hopefully we won't need to change the derailleurs when everything goes 10 speed next year…. 😀

will 9sp chain fit through 10sp mech?

Yes the 9 speed chain will fit. Shimano stopped making 9 speed Ultegra mechs as the 10 speed does the same job.

I currently Run a Tiagra Mech and 9Spd Cassette with an LX 9Spd shifter for DH, not the same bits I know but the same mix of Road and MTB bits, it works perfectly…

All shimano stuff pulls on the same cable ratio and sprocket pitches are the same for Road
and MTB so 7/8/9spd road cassettes will work with the corresponding 7/8/9spd MTB shifters, the mech just does what it’s told….

Tiagra Mech and 9Spd Cassette with an LX 9Spd shifter for DH, not the same bits I know but the same mix of Road and MTB bits, it works perfectly…

Not quite a true comparison though, as Tiagra is 9 speed.

It'll still work though!

Thanks for the confirmation and I'm glad that derailleurs are not self-aware…