This scenario would not be practical in this case, since the Bayern-Ticket is cheaper, but if you bought a ticket from Munich to Nürnberg for a connection that has a stop in Regensburg, you can get off in Regensburg and spend as much time there as you want as long as you finish the journey by the end of the next day. For example, the RE that leaves the Starnbergerbhf (tracks 27-36) at 13:44 makes a stop in Regensburg.
You could also define a multi-train connection between Munich and Nürnberg with a change of trains in Regensburg, using the stopover function. With that ticket you could spend an indeterminate time in Regensburg. If you specified a stopover of 30 hours on the ticket you could always leave earlier.