i do not know the exact term, pcfx does scales output to ntsc, each game switches resolution internally. Pia Carrots main game(not menu) uses 341/1024 dotclock mode-set this to 256 and the text will be squashed. Chip Chan uses also this 1024 mode, the layer behind the text(the dark colored) when you go to options menu or the back-layer of the high scores. set this 256/341 and there layer will flicker. mednafen does document these dotclock width to use 256/341 or even 1024.
According to the docs, the display output from the VCE only has 2 values (5MHz and 7MHz).
The 5MHz value is the same as the PCE's 256-wide mode, and the 7MHz value is the same as the PCE's 341 (aka 320) mode.
The effects that you're describing look like they are a combination of interlacing tricks, and how the VCE subsamples each pixel to determine which layer has priority (1024 times at 5MHz, and 768 times at 7MHz).
But that doesn't mean that your actual graphics on the screen have any higher resolution that 256-wide or 341-wide on each frame.
But you can display a *different* graphic every 1/60s and use a 0.5 pixel shift to make it look like a higher resolution ... a nice effect, but a lot of work for a small improvement.
The PC-FX developer docs are out there on the internet (in Japanese) if you do a search for them.