I started off just wanting square pixels, which requires the 6.5 MHz master clock to become 7.375 MHz. In practice, the baud-rate generator frequency 7.3728 MHz is near enough. 

This then requires modification to the software-generated video signal. 
When done, there is room for 40 characters instead of 32. 
This requires modification to the text routines. And plotting routines. I haven't modified the tape routines but who uses tape these days? 
I don't have time to check this out thoroughly, and it is unlikely that people are going to start writing code for a modified ZX80. Existing software will be messed up. 
However, I'm recording the achievement for anyone who might be amused by it. 
The photo is of an old 5-inch monochrome portable, modified to take baseband video. 
It would probably look better on a bigger, better telly. 

Will not work with ZX81 in slow mode, because the interrupt generator inside the ULA divides 6.5 MHz to get the frame rate.