I have been messing around with making hammer maps for a short while now, and I was thinking about how it might be interesting for a map to have both teams use the robot player models while they shoot at eachother in a space-themed map. And, since I have previously experienced custom maps such as those running the Stop That Tank! gamemode using the robot player models for the blue team, my curiosity was piqued as to whether this could be done in Hammer.
I think the model swaps in "Stop that Tank!" are made by the plugin, but it's possible to do it in hammer. I just don't know how.
I don't think it is possible in Hammer. There is in fact an input command called "SetCustomModel" which lets you turn any player entity into any static model (no animations). It's possible to make the model not rotate on player view, allow/disallow visibility of the model to the player, and changing the offset of where the model is on the player. More info here in the Player article on the VDC: https://developer.valvesoftware.com/wiki/Player (Just another tidbit of fun info: In MvM, you can make a robot player model into a human player model by sending a "SetCustomModel" input without a parameter.)