So first off please pardon errors as I am sending this from my phone. I am working on a project that the projection team decided to ignore my measurements and hang the projectors about a foot further from the screen then needed. Making my image about a foot to big. I am working with pre splits at 4320x1080 and need to scale them down to ninety one percent of their size. When I do this in watchout the image gets out of sorts. I understand why this is happening but is their a good way to do this without re rendering content.
I've never had to try this with pre-split media proxies, so I don't know how Watchout will react, but have you tried changing the display properties to modify the stage size of the displays?
If you keep the output resolution the same, but increase the stage size of the displays (to the reciprocal of 91%, so approximately 110% of their current size), the pre-split media should appear smaller within the display, without scaling or moving the media.
Of course, if you have any other media in use, this will probably wreak havoc with your show!