Are the OSX Mavericks tags visible from the command line?
Yes, you can find files that have a given user tag using mdfind.

Create a file and assign it a a custom tag in Finder.

Then go in a terminal; you will find it with:

mdfind "kMDItemUserTags == Math || kMDItemUserTags == Programming"


or 

mdfind "kMDItemUserTags == Math &amp;&amp; kMDItemUserTags == Programming"


See also -onlyin aFolder to restrict the search.

I didn't check how complex these boolean expressions can be, but these two examples work.

You can pipe the output to ls like this:

mdfind "kMDItemUserTags == mathTag || kMDItemUserTags == anotherTag" \
| while read f; do ls "$f"; md5 "$f"; done


Also, mdfind has an option "-0" 

    -0                Use NUL (``\0'') as a path separator, for use with xargs -0.


which can be practical with nasty filenames.