Even being on a roster and receiving money from a different NHL team?
TBH, I seriously doubt anybody would be petty enough to call them out on this by making a complaint to the NHL and/or NHLPA... in an Exhibition Game circumstance...
...And if it were to happen, who is to say that he couldn't be loaned for a conditioning stint?
Also we must keep in mind that EVERYBODY... the NHL, the Organizations, the NHLPA... everybody... knew that Pronger was hurt in a Career ending manner and was not going to Retire -- for a myriad of reasons -- and he was allowed to be on LTIR for the Flyers... and, while Officially on an NHL Roster, to become an NHL Employee... and also was allowed to be inducted in the HHoF without being Officially Retired... Playing in an Exhibition Game for the Fans and former employer IMO carries a much less forgiveness by all involved than has been give to Pronger over the past few Seasons, I would certainly think.
JMO