Also, some browsers are overzealous about reloading pages from the server, which could mean a stray click would reset your counter.

The only way I'd support this would be a positively-selected 'I read to here' flag/box/radiobutton. For the servers' sakes, a field in the user's account that just pointed at that entry would probably be best, rather than run the risk of stray 'I last read here' boolean fields all over the place. It would also enable a 'Go to the last entry I was reading' navigation option, though I'm not sure how useful that would be.