Sure they can do it. Question is do they care? I'm not convinced they even looked at the PGN specification, otherwise the PGN output this site generates wouldn't look the way it does now.
NAG support in game view
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Having the (already pretty robust) PGN editor and viewer be able to import, say, 12. Nc6 $1 and actually have it display as Nc6! instead of silently discarding all the NAGs would be pretty nifty. Other common symbols that have NAGs with visual equivalents (mostly coming from Chess Informator) and that the PGN viewer/editor could support along this line are $7 (□) (Forced move) and $16 (±) (White has a moderate advantage), among many others. The PGN standard has the full list, and the pgn4web project uses a special Chess Informator font (with permission from the company) to display those symbols -- if they can do it, I'd think that you people would have no problem.