Colorful line numbers

[João Távora]

Hi,

I'd like to play with fontified line numbers.

As far as I can read from maybe_produce_line_number in xdisp.c the face
is one of a fixed set: line-number, line-number-current-line,
line-number-major-tick or line-number-minor-tick.

IOW, there is no Elisp mechanism for changing the line number on a
line-by-line basis, for example with some text property set on any
character of the given line.

Could we implement one such mechanism?  Was such a mechanism ever
considered useful/practical/impractical?

Thanks,
João