> ETags can be used to track unique users, as HTTP cookies are increasingly being deleted by privacy-aware users. In July 2011, Ashkan Soltani and a team of researchers at UC Berkeley reported that a number of websites, including Hulu, were using ETags for tracking purposes. Hulu and KISSmetrics have both ceased "respawning" as of 29 July 2011, as KISSmetrics and over 20 of its clients are facing a class-action lawsuit over the use of "undeletable" tracking cookies partially involving the use of ETags.
It appears that there have been at least a few cases of this in the wild.
The main distinction (at least to me) between ETag and the other tracking methods you mention is that ETag doesn't appear to be easily clearable by a user (although that sounds like something browsers should fix if they haven't already).
It's unfortunate that features like this end up getting co-opted by trackers, which leads to breaking legitimate use cases like your app in the process.