The likely reason for Robert Urich not being in DS9 is that Paramount was really anxious about people seeing Avery Brooks as Hawk. That's why they made him shave and grow his hair for the first few years. It took that long for Brooks to convince them to let him go back the way he wanted to look. They where scared to death people would see "Star Trek: Spencer for Hire" and so, it's no surprise they never had Urich appear.
TPTB where always weird about stuff like that. They spent so much time fussing over Janeways hair and chest. Kate Mulgrew laments it at every convention, about how insane they where changing the hair and making sure her chest was neither too much or too little or this or that. Again, it took until season 3/4 for them to let her just be herself with such.
The only reason we had Stockwell w/Bakula for a Quantum Leap reuinion was the sci-fi connection and Enterprise needing every boost it could get, plus Bakula's behind the scenes power compared to his other captains of past shows (IE, Bakula was already much more established as 'name' and a 'leading man' in comparison).
They could have even had Sisko actually playing Hawk in a holosuite program. That would have been hilarious.