XMetaL Author uses Windows Script Host and supports any scripting engine* that is compatible with WSH.
If you can find a Windows script engine that supports EcmaScript 6 then it should be possible to use it with any software that supports Windows Script Host (XMetaL Author, IE, MS Office, etc).
The following scripting languages have script engines compatible with Windows Script Host. There may be more. These are just the ones I am aware of.
JScript (included with Windows as jscript.dll) — most of our clients use this
VBScript (included with Windows as vbscript.dll) — many of our clients use this
Perl (separate install – ActiveState makes one called ActivePerl) — several of our clients use this
Python (separate install – ActiveState makes one called ActivePython) — I'm not aware of any clients using this but I tried a version years ago
It may not make sense to try to find a separate script engine however. Microsoft seems to be suggesting they will be updating their scripting system to support EcmaScript 6. I suspect if they do it would be on their newest version of Windows. JScript itself has essentially been Microsoft's extended version of EcmaScript for many years.
* Microsoft has strict requirements for script engines, so if an engine is written according to those rules it should work, at least in theory.