1. Can the Harmony Hub be used as an IR Blaster with Media Center?
Yes. You can set any button up on the remote to blast using standard Windows Media Center IR commands, which MC will recognize without needing any third-party software.
If you want to control
other software on the computer (I control Firefox on mine, for example) then you need something 3rd party like Girder or EventGhost.
2. Is Media Center able to issue IR commands when playback is started/stopped in a zone?
No. Well,
it doesn't do that. The hub only "talks to" the computer. The computer doesn't talk back to the hub in any way. This isn't really needed, though. If the PC can blast IR, you can blast it directly to the devices, and the hub isn't needed. Basically, the hub is a way to translate the buttons you press on the
remote into "actions" on the hub (which might be Bluetooth, but are generally IR). In most other universal remotes, this function was built directly into the remote itself.
But having it in the hub means it doesn't move, and so you eliminate line-of-sight problems (and can easily wire up little IR extender nubbins). That's the main benefit.
There are downsides to the Smart Remote. The software is easy, but somewhat limited. It doesn't do things "conditionally" in essentially any way other than what "action" is currently selected. You can program each "action" (which are basically just modes for the remote) to do different commands when it starts up (and shuts down), on a per-button basis. However, you can't easily copy these sets of commands from action to action, so there is no way to duplicate and then slightly modify a setup (other than manually recreating it for each action). The remote also has no sense of "state" (other than what action is currently selected). So, in your setup, the remote itself would not, and could not, "know" what was happening on the PC.
The latter might not matter. The remote, for me, is just pretty dumb. It blasts essentially the same commands to the PC all the time (IR "representations" of the buttons I pressed). Then the computer interprets these differently depending on state (what app is foreground, mostly, but also what is currently happening).
It is not a total room control system. Just a remote, designed to get commands to the PC (and turn the TV and amp on). It is handy that it can also control other consumer electronics devices, though, without basically any reprogramming (other than selecting a pre-built action for one of the Action buttons). It works great as an AppleTV remote, for example, and I expect the same with an XBox or PlayStation.