It's hard to state if this would work from the picture, but if I was committed to removing the plug with the transmission still in the car, I'd be tempted to do the following:
Cut access hole, sized to accept an off-the-shelf plastic or rubber plug, as close as possible to the frame tube
Get square stock the appropriate size, then using a grinder, create a ball end that will allow you to insert it into the plug at an angle. Something like this hex key
Assuming that works, you may still struggle to remove the plug in the available space. I don't recall the length of the plug, but even if it is shorter than the gap, it may need to be loose enough at some point that you can unscrew the last bit with your fingers grasping the exposed threads.
Do you know how long it's been since the fluid was changed or the engine/trans was out of the car?
-John