Thanks for the explanation on CUDIMM, I never quite grokked the difference besides it being more stable with two sticks per channel. Hopefully they'll make an ECC CUDIMM at some point, but I'm not holding my breath.
If they don't and you are up for a challenge in bga soldering you can make them yourself if there is pad for the chips. You likely have to buy an extra module to get the chips though.
This would also need a SPD programmer and possibly some additional SMT resistors, but it is possible in theory.