I've never seen the monitor area of a dedicated MK opened up, but I have two Dynamo cabs, and they will do what you want perfectly. Since they were made with the intent of having a variety of hardware installed in them, they're a breeze to change around. On the 25" Dynamo I have, the monitor is mounted on a board with handles cut in it that you just unbolt and rotate. The board is cut so that it drops right back into the same bolt holes with the monitor in either orientation.
The decision you're going through was tough for me too. Most of the games I liked were vertical, but the real problem was that my absolute favorites, Robotron, Moon Patrol and Defender weren't. I eventually built one cab in each orientation, but got by for a long time just playing vertical games shrunk down on a horizontal monitor.