Yes, when I worked, we could request control of an etkt from any airline. I don't know what criteria went into why it was or wasn't granted, I only know we could do it and I have done it numerous times.
As for booking on UA, I did it on plenty of occasions. Couldnt tell you the year or when I did it last, it had been awhile so maybe more than 4 years ago. I've haven't worked for NW for over a year and half now. But I remember very vividly booking people on the morning LAX flights from HNL many a time when our late night flight went in the crapper.
I have no interest in asking any UA agent or otherwise about anything anymore as that part of my life is over, I'm happy to say. Started in 1986 and got out early with some sanity left.
BTW, if you're a current or previous airline employee, you are required to register that you are/were. I'm sure the admin will give leniency due to a bunch of new members but don't forget to do that soon. Here is the link. In case it doesn't work, click on User CP in the links that go across near the top of the screen and then click on Group Memberships in the Miscellaneous subsection on the left side.
Group membership registration