I tend to think that at this point we should postpone this to the next level of the transitions spec.
Though, actually, now that I think about it a little more, waiting to do this might lead to compatibility problems.
Also related to http://lists.w3.org/Archives/Public/www-style/2009Nov/0328.html
Wrt compatibility problems... could you make the default value of transition-property to be 'auto', and thus only transition interpolable properties by default? Then if you want non-interpolable properties to transition, you have mention them explicitly.
There is no 'auto' value today; since we want to push this to LC and beyond, shouldn't we avoid introducing new values and change initial values?
I think in http://lists.w3.org/Archives/Public/www-style/2012Nov/0261.html we decided to do this for CSS Animations but *not* for CSS transitions. Though the minutes aren't incredibly clear.