Of course that wouldn't be easy. But it could be done - with a fast '51 clone (there are ones out there that work with much faster clock), a few megs of external memory (AdUC812 supports 24-bit address space, there might be bigger), 2 I/O ports of some kind (lpt/rs232/usb, note if it was to interface a USB printer it would have to include a USB host chip) and quite a bit of pretty heavy programmer's work.

[rant mode]
Well, the task is difficult but not impossible. And as for the question "Why in hardware, when software solution is possible", I've spent tens if not hundreds of hours trying to configure different printers to work with PCs correctly, and way too often it was really terrible work - if I have a computer with 5 different operating systems installed (yes, real scenario, and it wasn't a pointless setup), I need 5 different drivers, if I reinstall OS I must reinstall drivers, sometimes the drivers make me curse loudly because of amount of junk included, often they don't exist for given OS...
One notable exception from drudgery with printers were the "postscript printers". They worked out of the box, all I had to do was to set up a generic postscript driver available on every OS possible, and it just worked. It was pure pleasure working with these.
Except that the comfort wasn't for free. I don't know what in your opinion is a "cheap postscript laser printer", but for me anything above €100 is rather much. What about the €20 home "toys"? How much would postscript option cost for these? And these cause most of trouble!
Maybe '51 is not the way to go, maybe it should be some DSP or something. But please, don't mention PC drivers.
[/rant mode]

Just for another nice idea to prevent some evil patents - what about a computer mouse with a trackball-style ball mounted where now the "mouse wheel" is? :) Instead of current 2D (mouse without a wheel), 2D+1D (mouse with one wheel) or 2D+2x1D (mouse with two wheels) you'd get real 2D+2D :)