For (my own) future reference, here's what works, showing that a circle's radius needs to be tweened within the attr property but stroke-width, stroke-opacity and fill-opacity (for example) don't:
<circle cx="75" cy="130" r="0" id="wave"/>
var tl = new TimelineMax({repeat:-1});
tl.to("#wave", 1, {attr:{r:100}, 'stroke-opacity': 0, 'stroke-width': 5, 'fill-opacity': 0});