It was rumored there was a mod or early firmware in the XTS3000 that did it, but I have never seen it personally. The 3K, and the ASTRO25 line have a bar-graph that only is used for trunking. The only one that doesn't would be the Astro Saber, AFAIK.
No, it's actually a receive signal strength indicator, which technically is the amount of signal present in the first mixer, traditionally. The value is used for conventional voting scan, so no control channel is involved in that case.
Also, the RSSI value is directly readable when doing front end alignment on a VHF or UHF portable, as they do not have fixed tuning RX front-ends and you can read the value while doing the tune-up. I just do it by ear anyhow, since it's too unstable jumping around to use as a serious tuning indicator. For more coarse values, like jumps of 5 or 10 it's suitable for doing voting scan or control channel scans, etc.
Thanks for the info everyone. It's stange that lower tier pro series offer this but the more advanced public safety series does not. My Waris family radios displays the RSSI when a carrier is present. Oh well....
They probabl;y havent included it because too many conventional users will be looking at the RSSIO rather than using the radio. Generally in trunking it is only refered to by the user to confirm poor coverage.