All forms of ID tickets are usually obtained through staff travel, although these days at most carriers it's much easier with web based ordering and ticketing etc.
In the past, I've often travelled on ID50's when I know I need to be somewhere at a certain time and the "nonrev lotto" game looks too risky. These days however, you can usually pick up the discounted web fares or similar for less than an ID50 anyway. (Example - I found a USD99 one way LAX-HNL on offer on HA's website a few weeks ago. Published fare, "pos space" seat.)

My best ID50 bargain was AUD700.00 back in the early 90's for a J cabin seat on EK from CGK-LHR via SIN, CMB and DXB. Of course it helped that fares out of Indonesia are in the very lower end of the scale, and I wouldn't have got anywhere near the same deal ex SIN or SYD.

If UA don't have many ZED's in place, then look at Multi Endorsed ID's, if they offer such a thing. That's one way to get around holding heaps of back-up's, but does require planning.