This display replacement looks great. I would be tempted to do away with the comma drive altogether to get the same brightness on the digits as the units seem totally usable without the comma's, I would do this either by removing C20 and C26 on the front panel or R35 and R45 on the digital PCB.
Before reading the cct description I though maybe the commas where using up some of the display multiplexing time for doing the comma's only and thus leaving less time for the digit display but I was wrong and it just appears to be the extra loading of having the comma on that is reducing drive to the remaining segments. Maybe the comma drive signal could be used to switch in a circuit to provide extra current but it seems less hassle to do away with the comma's completely.
Another though is maybe trying to reduce the current available for the comma LED's and see if the rest of the segments get brighter, I guess dimmer comma's would be more aesthetically pleasing than having dim digits.
Less than a 100 euro's for 2 displays seems a bargain to me if you consider the price of used Datron gear, ok, maybe not for the lower end Datron DMM's but definitely for the 4XXX instruments. Seems well worth the price to keep these nice instruments from the landfill.