I'm a big Evernote and KM user, and I like your idea. In fact, I've thought of doing a similar thing using @DanThomas' "Spotlight Search Prompt".
It would be best if you can post/upload the Macro you have so far.
But my initial thoughts are that you would need to use KM to select the Evernote Tag(s) that you want, and then have KM paste (or type) the properly formatted result into the proper field in Evernote.
I have a KM Macro that does something very similar. It allows the user to select text (from anywhere), or enter text, that will be used as a "intitle:" search in the Evernote Search block. The Macro selects/clears the Evernote Search field, and then pastes something like this:
inline:"users search text"
In your case you would use KM to build the proper search string for tags, something ilke this:
tag:myTag1 tag:myTag2
I would love to see how you are getting your tag list from Evernote, and then using that with @DanThomas' macro.
Perhaps together we can build a great KM macro for Evernote.