Depends how forgiving your TV is about the line rate. If we consider that the actual sweep rate of both the horizontal and vertical sections are fixed, then by upping the line sync frequency we can physically fit more scanlines in the same space.
A good estimate for the normally visible number of lines is around 216. Thus we need to increase the line rate by 240/216 = 1.111, or about 1/9th. That's actually quite a bit when we're talking about adjusting the scan rate of a more or less fixed-frequency display. For a regular colour NTSC television, that'd increase from 15.734kHz to 17.482kHz, which is most of the way towards MDA scanrate, and certainly beyond even the 16.5kHz of "enhanced resolution" arcade monitors (which are likely just normal TVs tweaked to their limits to fit as many visible scanlines as possible within a roughly 50Hz refresh rate).
I'm not saying you'll not find a TV that's happy playing along with that, but I think it'd be difficult. You're more likely to blow quite a few. Might have better luck with a proper computer monitor, as they're apparently somewhat flexible (there are a bunch of "extreme" Amiga modes which tweak almost down to 15.0kHz, and well up into the 16s, all allegedly 15.7kHz monitor compatible), but it feels a bit too much even so; a multisync would be better, but that kind of takes the fun out of it because you can just set whatever mode you like. And even with the regular monitor... you could just set a normal 240-line mode and then play with the picture controls.
The other consideration is that you'd have to shorten each scanline by 10%. That's maybe not so much of a problem, if with a typical machine we may have 455 pixels times per line, the adjusted version is still about 410, but it's eaten into our blanking quite a bit; with a 320 pixel line, we had 135 spare clocks per line, now it's only 90, and that could actually be kinda minimal for getting active area, border, blanking, sync, blanking, border and repeat. The deepest workable overscan tends to be barely any tighter. Or in other words, a line was about 63.5us before, now it's 57.2, and the usual full width (albeit with overscan) active period is about 53.3 (and visible, around 48us tops). The penny's been considerably shaved, and we may well find ourselves hitting flyback before the beam's off the right hand size (if we assume it always flies back to the same place ... which itself could be an issue for generating a biased AC waveform in the flyback/sweep circuitry and thus making considerable DC current), maybe even displacing the image to the left...
So in conclusion, whilst it's a decent idea, and could possibly be workable with a more modest goal (and, heck, I have a feeling it might even be employed to produce fake interlace on some recent Atari ST demos by hacking the length of several early lines on every other frame - as this is a system with NO native hardware interlacing ability, but the demowriters claim a true 720x440i colour resolution on a system only normally capable of 640x200p colour or 640x400p mono), to achieve what you want would almost certainly demand just too much of an everyday TV and cause it to go up in smoke.