well, i had tried that before but sulphur has very low solubility in water, so i do not know if works as any form of prevention. but when i dip those roots in the powder, basically is only the cut end. i did not observed any harm on the orchid roots by doing that.
but if you just want to "cure" the cut end of the root, dip in in any disinfected solution of your choice then just let it be for a day or two before repotting.
but technically, i prefer dipping them fungicide solution. i used Mancozeb, read a lot on use of Physan but i had never come across it here. copper sulphate and bleach for very badly affected plant but the phals hate them.