No way I could see it happening, unless he'd accidently killed or seriously injured a bystander in helping tosk escape... and wanted to avoid murder charges.... oh wait obrien would NOT just run away. Maybe the aliens DID try taking tosk through force... and keiko and molly both died? whatever... point is anyone pictured just for a second the thought of the two of them as inseparable comrades struggling to live just one more day perpetually. Though i know just the offer shows how much had formed between them, given how standoffish Tosk had been at first.
Since it is very likely that the Tosk and Hunters are members of the Dominion (they were actually going to put this on the show - and tell me that you don't think Tosk look like Jem'Hadar), perhaps it's just as well that O'Brien didn't go with Tosk.
LOL at Mr. Beam. I think it would've been cool - the places they may have gone and the things they would've seen. Though if the Hunters had later killed Tosk, I doubt O'Brien would've enjoyed travelling back with them.