Injecting current into the feedback node of an LDO/DCDC, bad idea?
Edit: The question originally posted proposed using a 3-terminal regulator. It was later edited to propose using a different type of regulator. I will leave this answer posted for future readers who might be considering something similar with a 3-terminal regulator.
Here's a diagram of the internals of the LM317 regulator:

Note: LM317 is a linear regulator, but it is not a low drop-out linear regulator. An LDO may have a somewhat more complex circuit, but this will mostly still apply.
The key things to notice are:

current must be allowed to flow out of the ADJ pin for the circuit to work.

The circuit maintains the output voltage at 1.25 V above the ADJ pin.

As typically used, the circuit has no connection to a ground node, so it is relatively indifferent to what happens at the ground node in your circuit.


You can use this to buffer a DAC by simply connecting the DAC output to the ADJ pin. In your circuit diagram, this would mean removing R2 and reducing R5 to 0 ohms. The DAC must be capable of sinking current. Then the regulator output will equal the DAC output voltage plus the regulator's reference voltage.

I aim to drop the output voltage past the regulator's voltage reference, is there any issue with doing that ?

You can do this by pulling the ADJ pin below ground.
If you try to do it by injecting current in to the ADJ pin it will more likely just make the reference circuit not work correctly.
A more common way to buffer a DAC output is to simply use an op-amp buffer, possibly with a further external transistor buffer if high current is needed. This eliminates the offset by Vref and associated errors.



simulate this circuit &ndash; Schematic created using CircuitLab
(Note: This circuit can only source, not sink, output current)