Great tip. I just pulled together a folder of all PDF's I authored by using "author:ted kind:pdf". Now only if I could do the same for meta tags in MultiMarkdown.
At the risk of making this sound simpler than it is, just use a shell script to parse out the tags, apply them as OpenMeta to the file using the openmeta CLI and then you can Spotlight them with "tag:tagname". I have similar scripts, but don't use meta tags much in MMD, so I haven't pursued that particular path.
Note that Spotlight search keywords, such as "category" change depending on the localization of your OS (which IMHO sucks, since, not existing a way to get a comprehensive list of what search keywords can one use in Spotlight, many times not even Google can help if your mother tongue is not English).
Remarkbox