Locking focus isn't a problem at all. It's more about changing the focus of the camera as the subject matter moves - and when you have a film holder in place you can't see through the camera how it affects the picture.
But I feel like it isn't worth taking it farther. I'm OK using the 5x7 whenever I see something that is a stationary subject, like the patch of cone flowers I have in my front yard. I took pictures of them yesterday with the 5x7. And last time I made portraits with the 5x7, out of ten exposures only two were sharp, because the person in front of the lens had moved ever so slightly, resulting in her cheek bone being sharp, as opposed to her eye. That 80% waste is too much for me, so I have chosen to not use the 5x7 for that type of subject matter anymore.
You're right - it's just a light and money trap. Thanks for bearing with me.