I want suggest that you create proxy daemon for RS, like MongoS for Cluster. It will be great feature, because it allow isolate applications from MongoDB infrastructure. Application will be know about local MongoDB (proxy instance). And proxy instance will be know how and where to connect. It very great isolation:

• programmers do their job
• sysadmins do their job, and if need made some performance/failover improvement they just do it, change proxy setting and no need to make configuration changes in application.

Candidate for such proxy already exists - it is MongoS. Just add ability to disable config servers and proxy for RS/standalone is ready

PS. Analogous solutions for SQL Databases

• MySQL Proxy - http://dev.mysql.com/doc/mysql-proxy/en/mysql-proxy-using.html
• PosgreSQL pgpool - http://www.pgpool.net/mediawiki/index.php/Main_Page

PSS. Possible options for mongoDB proxy:

• list of servers (standalone, RS, mongos)
• can handle authorization (keep auth settings), client can connect without login/pass to MongoDB which with auth
• slaveOk handle
• writing strategy
• handle QueryableBackups connections for MMS for restore
• log for sysadmins