I think I would offer that it appears to me that you are trying to 'not fix the problem, but fix something -near- the problem'.
Lots of research shows that you can not see through walls, but you can look around the wall or look through the window... removing the wall does cure the ability to see what is on the other side, as does going around the wall, or looking through the window.
in my opinion, you -need- to teach looking through the window, rather than removing all walls, [some walls are needed, they hold up the roof and windows frames, as well as block the wind and rain ;-) ].
I can offer that a standup narrow aisle truck like what is called a "counter-balanced-stand-up-end-control" will have the operator at a higher vision point, but as long as the operator refuses to correctly operate the truck, and/or is not properly trained and motivated, you will never be able to design away the mast.
As mentioned above, there is a mast we call a "trucker's mast", which is a very much lower overall height, but has a much lower overall lifting height.
The -real- answer {imho} is operator training that insists on teaching that "if your view is obstructed, you must drive with the steering wheels (not the steering hand crank, but the wheels and tires) in front".
I have always heard that, [in the forklift business anyway] "we don't try and build it 'idiot proof', since 'they' build a better idiot just the very moment you finally get it idiot proof".