Yes, they can be modified to do what you want.
As they sit, the input to the unit's internal power supply, and the voltage sensing input to the meter at connected together internally.
Separating them allows the unit to be powered from the 240V input to the variac, while measuring the variac output voltage via the meter voltage sense input.
Basically it involves disconnecting the large capacitor (or the resistor in series with it, whichever one it is) from the meter's input terminal and running it instead directly to the mains input.
I did exactly what you are looking at two years ago, and made a couple of videos about it:-
though I don't think I went into specific details on the mods required to the meter. I may have got them from this video, or one of several others out there covering the procedure:-
https://www.youtube.com/watch?v=wPFaEFEembc,But your meter is a different model, so you'll need to nut it out for yourself, though I'm sure the essentials are the same. (I've got one of your type of meter on order, so I'll be looking into this again sometime also)