Yes - in fact in CGA basically a NTSC-compatible 240p RGB signal with 262 lines per frame is already sent to the monitor, but because the rest of the lines are known blank, they are zoomed out on the monitor to fill the screen with only the 200 active lines. The same RGB signal is directly converted to composite for standard NTSC TV connectivity. In theory, all it would take is to adjust the vertical size and hold knobs on the back of IBM 5153 CGA monitor to slow down the vertical scanning enough to fit 240 visible lines instead of the faster scanning speed of 200 visible lines. The monitor itself has already locked on to the signal, so it is just really a matter of if the size adjustment range is wide enough to allow adjusting to 240 visible lines.
Please also note that when you say NTSC TV has 480 visible lines and 525 total lines, it means in interlace, so the display is scanned with 240 visible and 262.5 total lines in one 60 Hz field, so it takes one odd and one even field to scan the display twice with 480 visible lines.
It would most likely have been possible to extend the picture height of visible lines from 200 to 240 at least on an IBM 5153 CGA display. In fact, the IBM 5153 CGA display did come with vertical size adjustment behind the unit for picture adjustments.
CGA is rather compatible with standard NTSC signal. CGA uses 14.31818 MHz (4 times the NTSC color clock) master dot clock to generate the RGB video signal and composite output, so as CGA has 912 clocks per line, so their horizontal line rates are very close. The only difference is that NTSC is interlaced 525-line format and CGA is progressive 262-line format, so CGA has one half-line less per a 60 Hz field. So a CGA adapter actually outputs all the lines needed for a 240p signal on a TV, they are just blank due to memory not being enough.
The problem really is that while the NTSC video signal would have 480 to 483 active lines, or about 240 to 242 active lines per field, it does not mean they are all displayed as visible. Normal TV would overscan the image, so the visible area is horizontally and vertically less than the active image area.
That is why there is only 640 dots of active video per line on a CGA, plus border area.
So as the video width was fixed to 640 dots vertically, and that there was only 16 kilobytes of memory, it is only enough for 204.8 lines which would be guaranteed to be visible also on a NTSC TV set.
It would not have made sense to increase the amount of memory onboard the CGA adaper to get 240 lines, of which not all would be visible anyway. The frame buffer memory is dedicated video memory on the graphics card, so it can't be shared with system memory.
So to utilize the CRT screen area, they decided to not use square pixels on the CGA monitor and scan the 640x200 area to be fully visible (underscanned) in the 4:3 CRT monitor screen. The unused blank lines are still sent but zoomed out to fill the screen with only 200 lines.
On a NTSC TV, it would look slightly different due to more lines being visible in the same 4:3 aspect ratio screen area.
It might be possible to output less than 640 active dots, or rather, less than 80 active character clocks per line. It just leaves you with 68 character clocks, or 544 active pixels per line.