ok I got it to work, and my conclusions is you can't get it to do 50 col mode. There might be enough free space for patches but you then also need to expand some stuff in RAM and they didn't leave room, so it would probably break stuff.
The 128 has oodles of ram. Surely it's just about allocation? You would lose ram in one place to make room for another. Reduce the amount of basic ram? I am not sure where/how that particular ram is allocated but I would be VERY surprised if you couldn't jiggle it around to make it fit. Sadly, this would be quite a major change to the rom that "might" break some stuff, but I would hope not. You could always have it switchable like JiffyDos often is.
So you have the LNKMap which is 035e-0361 so after it is LATBL which is the logical file number table.. Before it is the TABMAP at 0354-035d which in order to handle the case where somebody pressed TAB it doesn't trash something, you would need to handle and expand, This is backed up at a60-a69 for when you switch the screen. While their might be places to place the extra data in the "shared RAM" you have to patch every access to these data areas, the copy routines, if you then need to move outside of the shared RAM then you need to set up FETCH/CMPFAR which takes up even more space.