Russ/Doug/Ian could surely offer additional products but none are interested. I think they just want a steady hobby during retirement and don't want to deal with growing into a larger business.
I think there could be a market for various divider circuits, along with a budget-friendly calibration.
The PDVS2mini actually doesn't particularly excel at mV values alone. For example, you could only do 1.00mV, although with a calibration performed at that point you might gain one additional digit that is only 1-2 off. Each batch sells out pretty quickly. I think the calibration set points can be flexible, so maybe check with Ian if mV ranges could be tightened up.
After thinking about this a bit, the one device that kept coming back to mind is the Advantest R6144 which can be found in quantity on eBay for under $500, primarily from Japan. It has gotten a varying amount of attention over the years. It can do 1.000mV for example. The UI is really nice and easy to use. I can connect up a known good DMM along with an unknown DMM and run through a lot of values quickly. Ship it out for calibration or do it yourself with a 6.5 or better DMM. The polarity relay is a known weak point, just stick to positive values and you'll be fine. You can expect the digit beyond what is visible to typically be around 1 digit off of zero, so for example if you select 1.000mV the actual value is typically going to be 1.0001mV, a least in my unit. It also has a great mA source which could also be used to generate precise mV values.