it seemed to me that he could simply put controlled current sources in the anodes, and sink the things with his ULN-whatever. The darlington doesn't care how large the current ... to a point ... and if he were to use a higher supply voltage, e.g. 12 volts, it wouldn't matter that there are voltage losses along the way. Whether his current is controlled with a mirror or with a fixed resistor to a regulated supply doesn't really matter. With a current mirror, however, it matters not what his Vce(sat) or other voltage losses are, again, within limits.

RE