An empty 2-color png has a size of only 116 Bytes so network-traffic should be also no problem.
Once you are below TCP packetsize it doesn't really matter.
Keep in mind that you will have to use a PNG transparency hack to IE users (also I'd go for a bit more than 2color PNG, since that will look crappy).
Rendering-on-demand sounds a good idea or we render on free resources off external computers.
Should be totally doable, I'm rendering all high zoomlevels in WMA on demand (with caching) and the WIOSM layers would be geometrically very simple.