[EDIT]
I found a solution (a few dirty but it works) : Solution
I’ve been very surprised (positivly surprised) in seeing Piwik include a noscript alternative (Google Analytics do not).
But there is no stat with Javascript enabled percent in the plugins page (or I have’nt found it)
However it would be easy to count how many visitors load the and how many load the (and the image).
If it doesn’t exist, I could try to create this plugin.
I’ve just tried with severale proxy web. &rec=1 doesn’t work ! I added &rec=1 at the end of the URL of the image in the noscript and when I load my page with a proxy, the visit counter does not increment, but if I uncheck the “Remove Script” option of the proxy, the visit counter increment normaly.