gistfile1.txt

For example, lets assume we implemented this as a "license" flag ( we
shouldn't, its just a convenient place to make an example because the
infrastructure is already well defined )

A "gentoo" binary could have:

LICENSE=" GPL-2+ Gentoo-Binary "

And an "Upstream" binary could have

LICENSE="GPL-2+ Upstream-Binary"

and then users could mask Upstream or Gentoo provided binaries based on
their preference.

Though this preference would only really matter in portage deciding
about "||( )" deps.

  || (
      icedtea-bin
      icedtea   
  )

If their accept.licenses says

  "-Gentoo-Binary -Upstream-Binary"

Then the default might be to build icedtea from sources instead of the
more natural option of pulling a binary ( in this hypothetical example
case at least )

But I personally feel such utility is not really necessary, and
mechanised ways of differentiating between "Official" and "Unofficial"
binaries is just opening a door for later problems.

Any mention of || ( ) should probably make an alarm bell go off.

And imagining a reality where 

  || ( 
   icedtea-gbin
   icedtea-bin
   icedtea   
  )

Could be a thing makes me scared.