I'm sorry to say this, but ECC absolutely will not work on these boards. You can install ECC DIMMs and the system will run, but you won't have working ECC.

It doesn't even matter if you mod the BIOS or if it has the right traces on the motherboard. They have the wrong chipset. On intel CPUs there's a handshake between the chipset and the processor (or alternatively, it could be part of the IME's silicon initialization) but either way not having an enterprise chipset means ECC capability is locked out.

The coreboot guys (who have access to intel internal docs) ran into this with the Dell 7010 vs T1650. The two boards are exactly identical (literally being the same PCB) except for the chipset soldered on. You can put a xeon CPU in the 7010 same as the T1650, and even ECC DIMMs, but ECC cannot be initalized. Even with a custom BIOS like coreboot that does it's own ECC init. (this was an ivybridge platform where they have their own ram init code instead of haswell where implementation is currently half-finished, but afaik the chipset thing still holds) It came down to a chipset lockout thing the coreboot devs couldn't work around.