@gerikson:

I'm sorry, I don't have an answer, I just want to say I wish more tools accepted real SQL as a query language ;)

But technically you can do this. The Lightroom catalog is an SQLite database -- try browsing it with Firefox's SQLite Manager addon, and you'll see it's pretty simple.

You could do this in Lua by writing an extension -- alas, I haven't gone through that step so I'm not sure how to do it.

If you don't need it to be an auto-updating collection the way smart collections work, but rather can deal with running a script every now and then, you could write an external tool to query the SQLite database the way you want, and create a smart collection xml file explicitly including the particular photos in question, and import that into Lightroom. (I needed to do this last bit myself -- see this question)

---

I just tried this select query and it works great:

 SELECT 
    img.id_local AS imageID, 
    kwdef.name AS keyword,
    img.rootFile AS fileID,
    files.originalFilename 
 FROM AgLibraryKeywordImage kwimg
 JOIN Adobe_images img
    ON img.id_local = kwimg.image
 JOIN AgLibraryFile files 
    ON img.rootFile = files.id_local
 JOIN AgLibraryKeyword kwdef
    ON kwdef.id_local = kwimg.tag
 GROUP BY kwimg.image
    HAVING COUNT(*)=1
      AND kwdef.name = ?

where you should either replace the ? with a keyword name (e.g. 'foo') or execute as a prepared statement. The GROUP BY clause does the magic: this groups the results by image, HAVING COUNT(*)=1 tells SQL to limit its output to images having 1 record.

This query probably won't work quite right for images that have stacks, in which case it needs to be modified to ensure all the JOINs are one-to-one (except for the keyword-image table)