Nope, close brackets prove not to be exceptional for this plugin.
@BETWEEN — Returns the portion of a string between two delimiters. ... (Exception: If you want to use a close bracket as a delimiter, escape it.)
C:\> echo %@between[[^], This is line 6 of some file.] TextUtils plugin: Missing comma C:\> echo %@between[, This is line 6 of some file.] 6