Programmable power supply
What you want is called a power supply amplifier or PSA in the business.  You are essentially looking for a power amplifier with a gain of 2.4, although it only needs to drive in one direction but has to be isolated.

Think of how a power supply works.  At some point there is a feedback signal indicating how high or low the output is and a reference that indicates what that feedback signal should be.  The controller looks at the difference and tweaks the output up or down.  You can control the output by changing the reference signal.

Isolation makes things more difficult.  Sometimes this comparison to the reference is performed on the isolated side with a simple digital high/low indication transmitted back thru a opto to the hot side.  The system still works as I described, but the reference is on the isolated side which is not where you want it.

This leaves two possibilities.  Communicating the reference to the isolated side and doing the comparison there, or communicating the actual output level to the hot side and comparing it to the reference there.  Both schemes have some merit, but I'd probably pick the first in a DIY project.

The best solution is to simply buy a PSA if you can find one with the right characteristics.

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