After 10 or 15 minutes, MC will give up, at which point you'll get an error message which details the file in question. As far as I'm aware, that's the only way to find out!
I know what you mean about that status bar thing though...
A long time ago I pleaded for the tagging status stuff to be given its own space on the status bar, I mean, why not have it over in the far right corner?
Even if you did find and unlock the file for tagging, the waiting message would just get replaced with a "Done saving tag changes" message that you have to somehow clear without changing your current selection. It's a pain.
I solved this here for the most part by preventing just about all tags from being written to files and relying on a good library backup strategy instead. I keep artist, album, name and date in the file, everything else is database only. I don't imagine that that is a solution that will suit many users, but there you go, it's what works for me.
I may go back to saving in files if the tagging status info is ever given it's own real estate on the MC status bar.
-marko.