Oops, the filter built-in the NES core is actually "HQ2X". Core is available, it's open source. The original author is probably specified in it. Of course it's far from what we can get with the other aforementioned filters but that's better that nothing !
Of course, all those are processed with pixel shaders and fast GPUs which is has nothing to do with the way filters are implemented here, but have a look at the kind of result we can get with emulators :
compare this default pixellized rendering : http://nuage.tallur.net/index.php/s/BfJv8ev4oth28JJ
with a combination for instance of scalefx and hyllian (not necessarily the best) : http://nuage.tallur.net/index.php/s/dngjpmEixKmuvVE
There are more impressive comparisons / combinations but I don't have them at hand right now