In SOLR, you can use a function query to query docFreq directly, shown here: http://wiki.apache.org/solr/FunctionQuery#docfreq,
q={!func}docFreq(tweet_text, kw)
Note, also documented on that page are function query methods to get tf, idf and termfreq, which may also be helpful for you.
This is probably less relevant to this question, in retrospect, but I'll leave it for the time being in case it is useful to you.
IndexReader.docFreq(Term) could get you what you're looking for.
such as:
reader.docFreq(new Term("tweet_text", kw));'
IndexSearcher.docFreq(Term) is the same thing, by the way.