I think the deal breaker, as far as mass-producing this really neat retrofit, is milling the LED displays to get the width down by 1mm and leave room for discrete colons and comma.
Throwing numbers out there...
OP used
Kingbright PSA39-21 0.39" displays about $1.15 each
1000-lot X 12 digits = $14
For a custom LED display, I imagine it's the NRE to get a custom injection mold made and the custom LED die mounts.
Say it's $10,000 and good for a 4 digit module, need 3 per multimeter and you sell enough for 333 retrofits, total 1,000 pcs. of a custom 4-digit module.
That would make the NRE $10 per (4-digit) display module, and then add the LED module's cost to that, say $11 ea. so $21 per module ($63 per DMM) and $21,000 outlay then.
Someone would have to hit up some LED manufacturers and see if my guess is far off.
Kingbright SMT LED display ACPSA04 is 9.3mm wide vs the original through-hole 9.9mm-1mm milling... I wonder if that would work? It's not as thick too.