Yes, an alternate syntax would require a lexer plugin. (Or getting a new lexer integrated into Scintilla, and then getting NPP to update Scintilla to the latest-and-greatest, which hasn’t been a priority of the NPP developer.)
Or, if you’re brave, I’ve got some links to some old posts by @Claudia-Frank , who had done some work on making custom lexers using PythonScript. Those links are in my last post in https://notepad-plus-plus.org/community/topic/16164/bug-javascript-multiline-character-not-understood-by-notepad/11 . It’d take some effort on your part to customize those to work with HTML+templating, but it’d be a starting point.