This is the way to do it, and the way GC and SGC do it since their CPUs work at a much higher clock while appropriately delaying the DTACK signal to the CPU will slow down it's bus operation.
7.5MHz clock would be provided to the ZX8302 for serial and MDV compatibility. IPC and NET are bit-banged so timing is entiraly software controlled, though I suspect there is some hardware support for NET but probably buggy so not used.
Running the CPU at 8MHz would also increase the video base clock to 16MHz and pixel clock to 10.666 which would, with appropriate line timing result in the full 512 pixels in the horizontal direction visible without overscan on a PAL monitor or TV, at least according to PAL spec.
There are also other possibilities once a FPGA is used instead of the 8301, given that the 8301 is the main memory decoder.
I would honestly not bother with the motherboard RAM, or at least the part containing the screen, rather include a 128k SRAM on board since this is much faster and lower power and still makes it possible to manipulate the existing 8301 based signals to make the access not slow down the CPU at all. Even better if the whole thing was moved to the CPU socket and using the 8301 socket just to pass the basic signals.
One interesting possibility is mapping some RAM in the 16k region starting at $1C000, which makes it possible to move the base of the screen to that address and have a 48k sized screen RAM for some extra resolution. Another possibility would be to use the internal FPGA RAM as a line buffer and get a QL display shown on a VGA monitor using a base 1024x768 resolution using a 2x3 pixel area on the 1024x768 screen to display 512x256 QL resolution.
That being said, if one can get access to 48k of screen ram, this could be extended to 512x384 using a 2x2 VGA pixels for each QL pixel. There are even more interesting possibilities - if 4x4 VGA pixels are used, 256x192 resolution will result but a 48k screen area affords us 8 bit color for each pixel in this arrangement, which gets you a really interesting resolution for games...