thanks to half an hour of coffee break, I implemented blargg's & byuu's average routine for hires/pseudo-hires pixels. once I have cleaned up a bit the code [1], this will be added as an option for the user, to more accurately reproduce the tv blurring the same way as bsnes does. MESS screen:

[Linked Image from]

[1] direct use of byuu's code does not work in this case, because we don't output pixels in order and I would like to avoid any additional drawing step (which could slightly hit performances)