Just to update, I have an experiment:

http://git.yoctoproject.org/cgit.cgi/poky-contrib/log/?h=rpurdie/overrides-convert

This contains a conversion script which can convert poky to use ":" 
instead of "_" as the override character (i.e. all of oe-core, meta-yocto, 
docs and bitbake inc. tests). I had to add in only two special hand cases
to the conversion code which is better than I expected (so far).

There is also a quick hack to make bitbake use the new override format 
(but no optimisation to the code) and a commit showing the result of the 
conversion.

Right now, this will successfully "bitbake -p", i.e. it parses. It will
even run a build but crashes out in gcc-cross do_compile.

You can see from the diff it generates, it isn't perfect and there are 
issues. At this point I should probably step back and finish the "diff"
tool I started a while ago so that we can compare to different builds,
see if (well, where!) the resulting output as parsed is different.

The point of this is to:

a) see if automated conversion is viable
b) see if the proposed syntax looks/feels better
c) allow experiments on things like performance to be made if I can
figure out where the differences are in the conversion and get builds working
d) if it can work, we could experiment further and see if there are follow
up improvements that can be based on this such as my previous experiments
with defval

So this is known to be rather rough and ready, it is very much experimental 
and brainstorming but probably worth sharing for anyone wanting to experiment.

Cheers,

Richard