Is it legal to use radios to communicate during a stalk, instead of flags?

Yes, there is currently no restriction against ground-to-ground radio communications.  Air-to-ground is unlawful. 36-1101. Taking of wildlife unlawful except by statute or commission rule or proclamation -- Methods prohibited -- Exceptions. (a) It is unlawful, except as may be otherwise provided by Idaho law, including this title or commission rules or proclamations promulgated pursuant thereto, for any person to take any of the game animals, birds or fur-bearing animals of this state. (b)  Except as may be otherwise provided under this title or commission rules or proclamations promulgated pursuant thereto, it is unlawful for any person to: 3.  Communicate from Aircraft. Make use of aircraft in any manner to spot or locate game animals, game birds or fur-bearing animals of this state from the air and communicate the location or approximate location thereof by any signals whatsoever, whether radio, visual or otherwise, to any person then on the ground.