depending on the material of the rear stay, you could possibly add bosses by welding some on as thats how they would be made in the first place.
but the issue i would consider most is whether the frame would be able to take the forces of a v-brake when it wasn't necesaarily designed for. that part of the stay would have to be strong enough to resist the rotational force of the wheel when braking, and also the pushing outwards force of the brake levers pushing against the side of the rim. You can visually see my steel hardtail frame flex outwards when you pull the rear brake hard.
An alternative if you have a steel frame would be to use a Magura rim-brake with the braze-on mounts. I guess you could get a similar mount machined in aluminium and welded on if your frame is aluminium and you know a good aluminium machinist/welder.
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