Ah, I see. I'm sure you are right. I just mistook your meaning. I might have said de-registering such a filter is very awkward or difficult rather than you simply can't. End result is the same, I got lost in silly semantics. End result is don't do it that way if you intend on (or someone else may want to) de-registering, it's not worth the trouble. Thanks for clarifying.
I don't think globals are such a crime in any case. Yes, they have issues, and we must avoid using them if there's a reasonable alternate. But creating tricky artifices just so we can pat ourselves on the back that we do not use globals is a bit of a self deception IMO. WP uses globals all over the place. A few more used very thoughtfully is not going to transform the installation into a complete mess. (at least no more than it already is ;) )
I've no doubt offended many coders who consider their code artful, in which shortcuts such as globals are as horrid as a garish inappropriate brushstroke across a painting. I understand, as well as appreciate, and even applaud such an approach. But they need to understand not everyone is talented enough, or even sees the need for such an approach. As long as my code does what it's supposed to with reasonable efficiency, is well organized, runs cleanly, and is easy to follow and maintain, I feel I've done my job, with or without globals.