I had the same problem, found out it is indeed possible to downgrade/flash stock RT-AC68 firmware (like the latest 3.0.0.4.380.7378) onto a DSL-AC68U with the latest proper firmware installed via the built in Broadcom mini CFE web server which is accessible via recovery mode.
It doesn't seem to care about file size mismatches and the like, unlike the ASUS recovery tool. After about 8 minutes the flash was successfully executed, the router rebooted, i telneted into it and erased the nvram via "mtd-erase2 nvram".
I logged into the web interface and checked, worked like a charm. The device still identifies as "DSL-AC68U", though DSL configuration tabs were gone.
Using the Firmware Upgrade Tab from Administration i then proceeded to flash the newest Merlin Release 380.65_4. Again, no problems or hiccups. The "downgrade" must have worked.
I am currently using it to power my SOHO and everything seems to work like it should, no issues so far.