This is a cool new placeholder, but I guess it's not perfect yet. 
For example it won't be reset under certain circumstances: after saving a IDv2 tag to APEv2 and then deleting only the IDv2 tag, %_id3v2_character_encoding% will remain displaying the charset of the now deleted tag. This is noticable when using this placeholder for a column in the file list.
I've noticed this instantly because I always convert IDv* tags to APEv2 in mp3s. Only after pressing the "Reload files" button, the "ISO-8859-1/UTF-8/UTF16" will disappear for the APEv2 only files.
BTW, what charset does APEv2 (and all the other tags... Vorbis Comment/MP4/...) use? Would it make sense to add extra encoding placeholders for these tags, too? I know it's especially useful for IDv2, and rather redundant for the other tags, but it might be a nice "eyecatcher".