Support for rates that low is actually quite common amongst modern LCD TV's -- even the giant 60"+ ones -- for one simple reason. It makes it easier to support old 480i broadcasts which uses the same pulse times as 240p. Same reason you can just make *nix modelines for 854x540p -- an unofficial / nonexistant mode -- and it works just fine.
In general so long as you get the VSYNC pulses 150 microseconds wide and your HSYCN at 8ms with a sufficient front and back porches it tends to work on modern displays.
Something I've been finding out by playing around is that the whole concept of "pixel clocks" is mostly BS used to make the math simple. So long as you have the sync pulses and porches the right width, and the correct number of scanlines for an existing mode output, you can make nearly any visible width you want on most any display. If you have full control over the "pixel clock" whilst outputting the horizontal data, you can even send 1920 wide to a CGA display... though the physical low ppi of the display makes it look as bad as 640 does on a color TV.
I was a little surprised to find that out... that the whole pixel clock thing is more fantasy than fact. Case in point right now I'm playing around using a Teensy 3.0 with a pair of SPI SRAM and a old Sierra RAMDAC to output 960x480 on a normal VGA analog and it works just fine on every display I'm throwing at it from an old mono IBM CRT really not designed to go past 640x480, to a 20" IBM CRT, to an older Dell 20" LCD, to my relatively new 42" media center LCD.
(the whole SPI SRAM thing is awesome, just switch them to QPI output and dump four bits each from two chips directly into the RAMDAC, all you need to do is make sure the driving clocks are right.)
Bottom line, if the sync pulses are within the displays rated range and of sufficient width, it tends to work... and modern displays -- pretty much all of them -- accept an absurdly wide range of horiz and vertical rates. LCD TV's often more so than CRT's thanks to the use of scan converters.
Just beware they usually take a full second worth of sampling frames to figure out what you're doing before they allow display after changing sync rates, and even the slightest hiccup in sync can cause them to go back into sampling with a blank display.