As seen in the docs:

http://www.mongodb.org/display/DOCS/Server-side+Code+Execution#Server-sideCodeExecution-Storingfunctionsserverside

runCommand can pass the 'nolock' argument to 'eval':

db.runCommand({$eval: function() {return 42;}, nolock: true})

But the shell helper for 'eval' can't pass options to the command, only arguments to the actual function being evaluated.