I recently got a HS-1 and it does not have a monitor. I have a couple 25" monitors in storage and wanted to check before I dig them out and see if anyone has even tried to fit a 25 in one before.
I have seen 25" monitors in atari cabs(dig dug). I have seen a 25" monitor shoved into track and field. It is done with L iron and sometimes losing the frame.
So I would say probably(especially if your talking vertical), but you may have issues with the neckboard hitting the back door.
Atari did somehting like that on its family cabs. Its about as small of a cab that can possibly fit (and rotate) a 25" monitor. They usually have wells k7000 in them. The frame would hit the back door so they cut out about a 2ft x 1.5ft square out of the back door and then added back a slightly larger square to cover the opening.. Here is a pic of the mounting frame. You can see the notched back door. To rotate a 25" mointor, you need to remove that frame and turn it 90 degrees. Its held in with 12-15 screws and 4 bolts.