It's still based on the availability of water, not the medium. The same amount of water and periods of drying is required by an orchid root no matter what media it is grown in. If grown in bark it needs to be watered often and will rely more on the humidity level. If grown in S/H it doesn't need to be watered at all, just keep the reservoir full.
Velamen cells rapidly increase in size when exposed to water. I suppose without searching for supporting evidence for it that the cell walls of passage cells in the exodermis could alter their water absorbing capability, but this sort of thing can happen in the existing roots making for fine adjustments in the availability of water. I doubt given the slow growth of roots that something is telling the orchid to grow a different kind of root because it is now in a different medium.
If you transfer an orchid into S/H does it languish until new roots are grown, not root tips, but whole new roots from the base of the plant?