DKB3128 cards would be very easy to make (especially as you could mount 128Mbyte on the card nowadays) but there is just no real market for them. Being Zorro III, A5k, A2k and A1200T's cannot use them. You might sell say 15 maximum. You're not even going to recoup the cost of making them in the first place.
For people with PCI bridge boards (Mediator/G-Rex/Promethius etc.), I am sure that you could add upto 512Mbyte RAM with a PCI RAM card (drivers for Gigabyte's I-RAM anyone?).
