The ISP policy is to do this over ISPERN, and often they will patch or console simul-select ISPERN to the various Dispatch talkgroups, this along with the regular ISPERN patches account for hearing the broadcasts on different SC21 talkgroups.
As for hearing 42.5 downstate, are you sure that was ISP traffic?
AFAIK they removed all the low band radios from the trooper's cars. When I visited the area a couple times this summer there wasn't a peep on 42.5 and all the cars I saw parked at Ullin and Carmi had no low band antennas. I occasionally hear skip on 42.5 from out East, North Carolina, I think, with 173.8 PL.