Also, what is not shown in the schematic yet is a diode-capacitor network to feed the control circuitry, which drastically improves the ripple rejection.
Such network (control circuitry capacitor) as shown on the drawn schematic ?
Fairly simple way to cut off the ripple from the main tank capacitor.
Using shottky diode may be better as long as there's a 30-40V zener diode in parallel to protect it from accidental reverse breakdown during shutdown or when taps get switched (main capacitor bank going much lower than the control circuitry capacitor). Most common diodes are rated for 40-45V reverse voltage.
However there still will be some 60Hz ripple as the diodes in the main bridge are not equal as they have slightly different voltage drops.
It's not going to be much but any RC filter for the control circuitry supply should be tuned for <60Hz rejection.
Another small reason to go with boosted voltage as there's headroom for filters on the control circuitry side.
If active current source is to be used the boosted voltage will be even more handy.
As for current limiting using NPN transistor.
Put the BE juction directly across the current shunt.
Remember about BE junction beeing low voltage zener which can conduct in reverse.
When applied across the diode it won't be protected in any way.
In this case the current limit will be dependent on not only the temperature but also the current.
Diode drop will change in roughly 0.5V to 1V range which is quite a lot.
Extra source of error is the collector-base leakage current (disconecting the base pretty much made the transistor conduct all the time, shutting down the supply for good).
For silicon this is fairly low but with germanium I had far much better results when adjusting the potential of the emitter instead of base.
Far less dependent on the voltage put across the current limiting transistor.
As for current limit adjustment.
Either have the main shunt made out of few smaller shunts in series and switch the emitter (base will generally work too, but switch resistance should be low to minimize the influence of collector-base leakage current) between different points using CD4051 (digital control) or some mechanical switch.
That or add schottky diodes in series with the emitter.
Either one works.
Resistive dividers over the main shunts are, even with silicone transistors, rather tricky.
My bench power supply uses 100ohm potentiometer over the main output shunt and I can easily tell it's nonlinear.
Classic 723 circuit with default internal current limiting transistor used.