The refrigerant distributor. I plan to use a few pieces of 0.8mm / .031" I.D. captube to distribute refrigerant over multiple 1/4" evaporator coils. I still need to find out how long I should make those tubes, but a rough guess would be 30cm / 1ft. I just need a bit of pressure drop to obtain a decent distribution of refrigerant, but how much pressure drop I need: I don't know.
Since the Prof has done a fair amount of work modeling distributor tube pressure drop, he should be able to help in this area...

When sizing a conventional refrigerant distributor, it is a good idea to size the tubes such that a pressure drop of about 10 psi is created which helps distribute the flow. Both 3/16" OD and 1/4" OD tubing are commonly used for distributor tubes, though with small capacity refrigeration units, one normally does not get much pressure drop with this size tubing.

In DaBit's situation, if we assume a -40°F evaporator, 100°F liquid entering the TEV, an R-507 refrigerant flow rate equivalent to 150 watts, and let's assume we have 3 circuits each being fed by 12" of 0.031" ID cap tubing. The Prof's model would predict a 70 psi pressure drop across this tube. This is much higher than desired. One must keep in mind that flow leaving the TEV will be two-phase, and that is why cap tubing isn't used as distributor tubes unless it is also acting as the expansion device. Using cap tubing with a 0.064" ID, however, should work ok in the above scenario, though one would want to verify pressure drop under the high load condition.