Nice! I like the interface.I've ordered some MC1566L 's and considering toroid transformers for the final version .Also I'm going to add regulation at the Aux ,mode indication and short protection with indication. Might even add a 5V USB just the the heck of it.
Yes, I prefer to do so too. One of my earliest LabPS was a mod of a discrete ITT application where I fitted a 3-transistor negative regulator to the aux voltage - it helps with drift, noise and tolerances.
On the topic of mode indication/short protection: I like those circuits where you can select from the classic LPS action (CV/CC) or a electronic fuse action, that trips on limit and needs an explicite reset.
Beyond this I have thought about integrating a current peak indicator mode.
Playing around with such add-ons becomes easier when one normalizes the measurement signal to 0..10V or other convenient range.
Also, I would add a thermal protection for the transformer and the power transistors. This could be done by thermoswitch, but thermistors would make it possible to have a dual warning/cutoff action.