While this doesn't solve all your proposals, you can somewhat do what you're wanting with a small alias using picture windows and not need "thousands" of image files.

This is just a rather crude and quick example just to show the concept... it'd need some work to mimic /background switches entirely... Probably will fail with things like "-t #mychan" for example. I only did this as a quick test with the common switches: -amsdluhb

Also: you'd need to save them somewhere with a unique name or always generate these on startup/window opening/etc... for it to be permanent since the temp file will no longer exist that is saved in mIRC's INI file under the [background] group.

Code
;/rgbbackground <target> <rgbcolor>
alias rgbbackground {
  ;Create a borderless 1x1 @picwin
  window -hpf +d @Pixel -1 -1 1 1

  ;Draw a color and save the @picwin as a PNG
  drawdot -r @Pixel $2 1 0 0
  drawsave @Pixel PixelTmp.png

  ;Use /Background to place temp PNG as background
  background $1 PixelTmp.png

  ;Delete temporary PNG
  remove PixelTmp.png

  ;Close our drawing surface.
  window -c @Pixel
}

Example of your first RGB color tiled on the status window:
//rgbbackground -st $rgb(59,62,69)