I was using Mamiya 645 cameras for landscapes but found I really needed tilt for some shots, I looked at a MF field camera option but I'd already been using 5x4 for over a decade for work and had an enlarger so went down that route. My De Vere monorail was too heavy and large outside a studio as it was a Whole plate camera which also had Half plate and 5x4 backs, so I went for a Wista 45DX.
Like you I sometimes want to use other lenses, in my case usually a wide angle alongside a standard. At first I thought my Busch Pressman might be a MF option, there are occasions when I like to shoot MF, need to travel light and want something more than my 645s which I rarely use these days or my TLRs all of which have no movements. In the end I built myself a 6x7 field camera using some Graflex parts I'd acquired - focus track etc from a Quarter plate Speed Graphic, and front Standard from a 5x4 Pacemaker Graphic.
Sometime ago I planned to convert a 9x12 Patent Etui to take inter-changeable lenses, the idea was to use a K mount I'd kept from off a scrapped Pentax body then attach K mount adapter to each lens. That would work with the Busch Pressman as well, it's not a new idea it was used on a few pre-WWII 9x12 cameras (obviously not K mount though). I've seen pages on the net showing this done some years ago.
Ian