In order to fit a foot plus a liner, all of these boot shells are quite a bit larger than the shoe size indicates. If you can get a size 13 shell with a thinner than normal liner, and preferably heat-moldable liner, you may be able to find something with enough space.

If you were to look into stretching the shells, I think you'd be better off with Fogman or FM E-Series boots as both of those don't have the boot in direct contact with the release mechanism. This might be a non-issue though.

If you are going the fogman direction you could buy bits and pieces from Fogman and find your own shells. There have been several discussions over at thewaterskiforum.com about using rolloerblade boots to make bindings like Powershells, you could do the same with Fogman release units and binding plate.

I feel like a large part of the fit and function of hardshell boots is due to the closed toe. If you have a bootfitter that would, or can, stretch the shells, I would want to go that direction first instead of lopping off the toe.

We can do custom work for you. Our XL boots can support Size 14. Width would be the thing....Go rear open toe for sure. Even the front open toe will work nicely. 

Normally we open the boots, and leave the  liners closed. But last month I cut the toe out of my rear liner in desperation in Cancun ( the heat caused my feet to swell arrgh!) I was totally surprised I really like it, have my toes there relaxed on top of the ski feeling the water. I want to trim the open toe liner and make it an official option. Going to suggest it to JT too. I feel more in tune with the ski.