FFGL can be limited in that way, but isn't a bad idea to have some color correction tools bundled-in (I can make my magic with just curves! and hsl kind tools don't need more than some sliders).
What about a simple LUT loader? Edit and make luts can be a heavy task for programming from scratch, but what about just avoid that (I think the use of luts to filter images is a wrong fashion trend) and have a lut loader for the purposes they where created? Every time you need to play footage shooted in flat profiles needs to apply a lut before reencode to dxv, and now with R6 taking a lot of compression formats a simple lut can make your day