Sorry mate, the FARs and UA's Company policy are very strict on this particular request. You may only visit the flightdeck whilst the aircraft is at the gate but once the flightdeck door is closed, no one is allowed in other than flight crew or cabin crew on duty.
Yeah I know it's BS but as the foreigners say here, "Only in America !"
It is a "security" thing so I'm sorry to be the bearer of bad news.
Fly Safe
------------------
Live Fast, Play Hard !