This seems overkill; why would you need to track more than 2k urls? Plus the iframe solution is not really something we are ready to implement (not industry standard, more a hack).
plus POSTing data is meant to be an "action", something the user clicked or validated.
Thanks for feedback anyway
With the new piwik.js, we might consider using XmlHttpRequest (or IE equivalent ActiveX objects) -- for Web 2.0 goodness -- and JSON. This would permit more complex data for custom_vars and allow for non-image responses, eg server generated idvisitor.
In , the XmlHttpRequest is now asynchronous.