It's a good idea and somewhere (not on AS) I have seen something very similar. They used round tube instead of square and were using a rail mill instead of an Alaskan + guide board which made getting the mill on and off a lot easier.
The play in the square tube could easily be removed by a serious locking bolt.
The way to get the mill on is to have a short board clamped across one end and put the mill on inside section of that board. Then slide the longer board through. I have done this a few times with my baby milling rig (see link in sig below) and it works OK
Your will definitely need support along the log under the board but that should be no big deal.
A
digital angle finder is a very good instrument to use to detect twist in the guide board. I ZERO mine at one end and then take it to the other and adjust the log rails etc until the angle reads zero there as well
With my big mill I sometimes have similar problems in dismounting the mill in unusual milling situations like this.
Here one end of the log dips well below the other end meaning I can't just sit the rails on the log so I use what I call my "vertical offset ladder", You can see the chain will exit the log well before the end. Getting the big mill off the rails at this point is a PITA so I usually just get someone to help me. If I am on my own I leave the bar in the cut and crank up the height adjusters on the mill. This lifts the mill clear of the rails and then I pull the log rails thru - awkward but not impossible.
Anyway I reckon you should persist as it is a good idea in principle but as usual the practice is when any problems come out.