My idea to just use it in the same way as it's already used:
gsap.timeline({
defaults: {
ease: 'circ'
}
});
tl.to('#a, #b', {
keyframes: [
{ ... },
{ ... }
]
})
I think both approaches have their upsides although I would be reluctant to add more special properties given the framework already has syntax to handle multiple targets.
I do like the fact your suggestion supports from and fromTo though