on demand is presently handled from the agent screen. turning on recording "for a while, for training" could still be handled by changing the campaign recording settings.
it would probably not be particularly difficult to create a "record this call" button for the realtime screen, but unless you really want to pick and choose specific calls already in progress, it is much easier just to change the campaign settings and then change them back after you've captured a few recordings. or any of the other categories of call routing that can be set to record. recording is in Many Places. LOL
it would certainly make a great feature request (by putting in a Vicidial Issue Tracker Feature Request) and/or a project for The Vicidial Group or PoundTeam Incorporated as well.