Interestingly, there are LaTeX packages for coffee stains[1] and to simulate the variability of an actual typewriter[2]. I suppose both of these involve discrete objects (the coffee stain, individual characters) and photocopy/scan effects involve 'fuzzing' of the whole image, rather than any particular element, and TeX isn't really set up for the former.

[1] http://hanno-rein.de/archives/349

[2] http://mirrors.concertpass.com/tex-archive/macros/luatex/lat...