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.