You’re looking for a device that is able to generate unlocked “standard” refresh rate AND buffer 1-3 input frames. OSSC as is can do the former (e.g. test pattern) but in no way the latter as it doesn’t have external RAM. The FPGA only has ~500 kilobits of internal block RAM while such framebuffer would require a 100-200 megabits if all supported input resolutions (up to 1920×1200) were considered.
It’s unfortunate that VP50 and Framemeister cannot handle transitions without resync even though they should be technically equipped to do so. Along with support for 4k and HDMI sources, frame buffer would certainly be high on the wishlist for a next-generation (gaming-focused) scaler. Scope of such project would be much larger than OSSC, though, so the bar is high for hobbyists. With right people and resources it might still happen someday, but don’t hold your breath.