Not yet. Other people have. Budget Multimeters › Aneng AN8002 Multimeter Review
The data sheet for the DMM chip briefly describes the calibration process.
Besides, Good trimpots would be too expensive for a $9 meter. 
My “old technology” DM200 has three multi turn trimpots. If I can find a schematic or manual, I’ll adjust for 2.500. Maybe. I’m not convinced quality trimpots are bad in a 2000 - 6000 count meter. My Tech 310 was only 1 count off after 30+ years. The DM200 is still in-spec after 5 years. I’m not sure if the ZT102 will last 5 years. It might.
The Penta Ref must use a programable precision voltage divider. I looked at this briefly with the idea of building a precision divide by 10 amplifier. My 310 is supposed to be calibrated at 250mV.
Agree. That’s why I don’t recommend buying a new DM200. Over on eBay a Tech 310 is $60 !! I’m happy to keep mine, but I can’t see paying $60 for a used one. There is also a pair of slightly newer 3010’s (only one works) for $75!!! Insane.
I’ve been thinking about buying one of the better Uni-T DMMs. 171-B (40,000 / 60,000 count) .025%+5 counts @ 5-times per second is £147 on their Aliexpress store.
I’m also considering a Vici VC8145 bench DMM. It is an 80,000 count @ 4 times per second. DCV 80mV/800mV/8V/80V/800V/1000V ±(0.05%+5)
Resistance ±(0.05%+3) It appears to be a bargain at $152.
Edit: Almost forgot. Dave Jones / EEVBlog / UEi 121GW. Waiting for details on regular production.