Page 1 of this thread has some useful info, but as for custom tweaking, it seems to be a little complicated regarding what a TV will accept. Sometimes it can be easier to use a computer monitor that accepts anything for testing, then try it on the TV when the monitor is reporting the desired resolution.
H Active will affect the horizontal resolution, V Active will affect the vertical one, but H. Samplerate can affect both the width of the picture and the resolution. Basically if you have V Active 240 and you’re using Line3x, you’ll be outputting 720p vertically, it’s usually a combination of messing with the H Active and H Samplerate on fussy TV’s.
A lot of TV’s have PC modes, so increasing V Active to 256 makes the window be 768p, and most TV’s will accept 1024×768, 1280×768, 1360×768 or 1366×768. The problem is that it heavily relies on getting the right combination of H Active and H Samplerate as to whether the TV will display an image. The biggest issue however is that most TV’s only use these PC modes at 60Hz.
There are 2 PAL tweaks on the first page and the one I used for 4x is great if your TV accepts it, but the TV also has to be able to force 4:3 since it shows it as a 16:9 window. There is a 1440×1080 setting that you could try, but a lot of TV’s are too fussy to accept that. My Panasonic was so fussy and I’m surprised you even got a picture in 3x with V Active set to 240.
This is what I had figured out while messing with the Panasonic before regarding NTSC here.