Have any Senior Staff NCOs ever had to pull aside and speak to an "overeager" new 2nd Lieutenant? Did your CO back your actions?

It happens all the time -- LTs need mentoring too

Happens and "CO Backing" isn't the point of the spear; it's the base. Decent Skippers set the tone and policies. Developing JOs properly is one of them. Typically the XO is responsible to running point on this. So if there is a Crusader Rabbit, word gets to the SEL who lets the XO know things are being taken care of. XO usually follows up with the JO's chain because a proper NCO intervention isn't a good thing if repetitive. That side of the CoC had better know what's going on. This knowledge is part of what goes into the mill for FITREP time where all the officers are ranked. For sure, if I got caught cold with an O-1 going into Crap Mode, then there'd be others in my office.

Maybe bring in a senior CPT to just sit down for a quick "fireside" chat, and mentor him. It's not necessarily a bad thing, just need to teach him how to focus his enthusiasm.