Had a play around today. Of the 240p games I tried, they to fell into two categories:
Sonic Jam (for example) ran at 320×240. Altering H.Samplerate (and sampling phase, but that’ll be personal depending on what LPFs you have in your chain etc) from the stock optimal 5x mode settings to 427 was all that was really needed. You might need to also alter H.backporch
NiGHTS seems to run at 352×240, which meant altering the H.Active to 352, then H.Samplerate to 455 (as Harrumph posted above). I used the clock text in the top left corner of the title screen to find the upper/lower boundaries of a stable sampling phase and then stuck it in the middle. Would have been better if I had a central point to work with but it’ll do for now.
Apparently it supported a load of resolutions – so I might need to make a few more profiles (https://segaretro.org/Sega_Saturn/Technical_specifications#Resolutions) but we’ll see once the 240p test suite hits.