That won't work. you need to handle those sequences, <special> and <post_include> at the same time, not before (which would break "&lab;special>lab&lab;/special>") or after (which would break "<special>lab</special>"). I've covered what would work elsewhere.
But I can't think of any reason at all that you would need those sequences in the course of developing an aXML based system! They are pretty much for the plugins only, and the only use I've found for them so far is outputting complex litteral strings which contain aXML/Perl... a feature I've never needed until doing the PerlNights project.
Look, why don't I email you over a copy of the the PerlNights build as it stands right now, and then you can have a look at a proper working aXML system that renders an output very similar to PerlMonks. Then you can see why I never really gave the specials much thought at all, because I never needed to.
I was going to test <special> and &lab; and interactions between them, but aXML doesn't compile.
$ perl -we'use aXML::Engine;'
Can't locate Conf.pm in @INC (@INC contains: ...) at aXML/Engine.pm li
BEGIN failed--compilation aborted at aXML/Engine.pm line 4.
Compilation failed in require at -e line 1.
BEGIN failed--compilation aborted at -e line 1.