I personally think that for a wild bun, the stress of picking it up and carting it off to the vets would be overwhelming for it and cause it more distress perhaps than just leaving it to be picked off quickly by nature's own methods which tend to be very quick....e.g. fox, buzzard etc. Wild buns are not used to people or being handled. Then how would the vet euthanase it? I doubt they would be able to hold bun still enough to catheterise in the ear vein so it would have to be a conscious cardiac/liver injection??
Unless you are fully prepped yourself with a change of clothes, disinfectant etc. you then risk transmitting it to your own buns through contact of discharges/fleas etc.
I HATE seeing Myxi buns in the wild, it brings me to tears everytime, it is a cruel and painful disease that we as human beings have inflicted upon rabbits, but I too would walk on by for all of the above reasons. I have rescued plenty of wild animals in my time, but I feel that there is not anything that can be done for a wild Myxi bun without causing it even more distress and risking my own buns in the process.