Laing D4 (aka mpc650) pcb mod
http://www.honigtopf.mynetcologne.de/D5internal.JPG
It looks like the Laing pumps have some sort of DSP drive. In the detailed picture, the VPP, DAT, and CLK testpoints means that it is most likely a 16 bit dsPIC. Not as powerful as the 32 bit C2000 and ARM processors in the Delta and Nidec fans, but still plenty of power to do good motor control. It looks like it should be pretty easy to tap a wire to control it from a microcontroller, but I'm not sure what kind of control range to expect. (Sensorless drives generally have difficulty at very low speeds.)
Assuming 5200BTU/hr of heat dissipation to the water (stock rating of A/C, would expect more after mods), 70F inlet water, and 140F outlet water (dishwashing mode), it translates into about 0.15 gallon/minute if I did my calculations right. The high pressure variable version of the Laing D5 seems to vary down to about 1 gallon/minute so it looks like the real problem might be the pump not being able to throttle low enough. I can easily workaround that by adding a bypass valve. There will be some efficiency loss but it would be tiny compared to the rest of the machine. I could start out with a bigger compressor but it seems wasteful when I would expect a 5200BTU/hr unit to easily keep up with all of my needs for hot water during the summer.