So the F(b9) chord would be F - A - C - Gb, but with the C on the bottom and then then notes played more like a flat-2 than a flat-9: F - Gb - A.
But I think what he probably really wants is a dim7sus, which is not supported (and which I have not heard of before). That would seem to result in C - F - Gb - A (The dim7 chord would be C - Eb - Gb - A, but the 4 (F) would substitute for the 3rd, so that's where you get the F and end up with C - F - Gb - A). But I feel like I'm making stuff up now.
No, you're not "making stuff up", you've absolutely nailed what I'm after right there, a "dimsus"!

I too only saw this "dimsus" thing for the first time myself last week, on the choir sheet I've been asked to make backing tracks for. When I then came up short in BBox, I resorted to looking at the notes being sung in that particular bar, and they were those four notes I listed above. My old auto-accompaniment keyboard listed those four notes as a flat 9 added to the F major, hence my initial question as I thought finding that would be easier...but apparently not.
So now we 'only' need to figure out how the heck I'm actually gonna
do it
