Domino Data Service is a good thing but is it possible to search for documents by key. I didnt find anything in the api and the url parameters about it.
You would do something like the following:
GET http://HOSTNAME/DATABASE.nsf/api/data/documents?search=QUERY&searchmaxdocs=N
N would be the total number of documents to return and QUERY would be your search phrase. The QUERY would be the same as doing a full text search.
For column lookups it should be something like this:
GET http://HOSTNAME/DATABASE.nsf/api/data/documents?sortcolumn=COLUMN&sortorder=ascending&keys=ROWVALUE&keysexactmatch=true
COLUMN would be the column name. ROWVALUE would be the key you are looking for.
There are further options for this. More details here.