Again, I don't have an SE/30 technical manual handy, but are you certain the video output and the CPU/system clocking are (or necessarily have to be) synchronous in the SE/30? Certainly not saying they're not, I honestly have no idea. I suspect there would be multitudes of fallout from simply "overlocking" the entire board; at the very least I imagine the disk controller and the serial interfaces would no longer work properly. The Motorola 68000 series, particularly the later models (68020 and up) use some sorta-complicated bus contention signals to mediate access to RAM so even *if* the pixel clock/VRAM timings are directly slaved off the same crystal that the CPU/bus clocks are derived from it would probably be *possible* at least to sever it and drive the video section off a different clock, although doing so might require some changes to the memory controller in the system... which is suddenly turning into a really ugly set of possibilities.

Honestly I have to wonder if it would actually be "more reasonable" to build from scratch a single-bit PDS video card with, say, 128k of RAM (Which would let you push resolutions as high as 1152x864-ish) than trying to hack what's there. In principle such a card should be "pretty easy" from a design standpoint, since the SE/30's PDS dispenses with the need to do Nubus arbitration, but from a practical standpoint the speeds involved put it into the "iffy" category for a home build. Just spitballing you could, for instance, use something like a Propeller MCU to generate the video timings/coordinate a couple binary counters to behave as a CRTC video address generator and implement the rest of the card with little more than a couple high-speed SRAMs and a shift register. Then you'd just need to write a driver for it and embed it in a ROM per how PDS cards do their magic and you're off to the races.