Chuck Clark said:
Not at all realistic. It takes the whole mind/body system .... the connection can be achieved and maintained through one finger though.
I agree with Mr. Clark about connection via one finger, but voted 'completely realistic'. I read the question to mean 'using only one's finger' rather than assuming it implied a finger wriggling detached on the mat.
As an example - perform a good irimi from shomen uchi and step fully behind uke - hook the collar of their gi at the neck and bend your knees. If you have their balance it works nicely (as I'm sure everyone knows)