Search for character that doesn't have a combining character?

I'm working on a custom command to add strikethrough to text, using the Unicode COMBINING LONG STROKE OVERLAY, 0x0336.

In this command, I want to apply a strikethrough to a character, only if it is not already present.

This pattern fails because it doesn't match *anything* with regexpengine set to 2, it does not match an unadorned character immediately before a struck-through base character, and it *does* match the last combining character in a word for some reason:

[^\u0336]\%u0336\@!

This pattern also fails, because it matches already struck-through base characters for some reason (although it does the same thing in both engines):

[^\u0336][^\u0336]\@=

What is the correct way to do this?

Full command (attempted):

'<,'>s;\%#=1\%V[^\u0336]\%u0336\@!;\=submatch(0)."\u0336";g

Note, how I'm also limiting to a visual selection; so I'm trying to use the :s command for simplicity.

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