There are some instances where rescued reptiles have been found to have snake mites (Ophionyssus natricis), but these have been in urban areas, and it is likely that those populations remain fairly confined. The chance of them being picked up and transferred to enclosures on branches is so unlikely as to be of no concern. The mite does no breed on snakes but needs to feed on snake's blood to grow into adult mites, and their life is not very long - if they don't feed, they die, unlike ticks which can go months or years without feeding. They are pretty fragile in fact.
They are believed to have originated in Africa and been spread throughout the world as reptiles were moved around by us, but in this country they remain largely an artefact of captive snakes.
Jamie