In the context of MCML...

>> Can I composite moving Text over video?

Yes. The Z sample application in the SDK demonstrates how.

>> Can I composite ActiveX objects - i.e. does it support windowless controls?

No. You cannot use ActiveX within MCML.

>> Is there anything that is likely to cause me problems when using multiple displays (all running different MCML pages or the same one)?

Yes. There will only be a single instance of Windows Media Center, therefore you can only have a single page of MCML. Also, if you run Windows Media Center in full screen, it locks the input focus (mouse, keyboard HID events) to that screen.

>> Can I composite video (WMV etc) that is either loaded locally streamed over the network?

Yes. The <Video> model item is not aware (nor does it care) about where the video is sourced.

>> How does DirectShow/DirectX fit into the whole MCML architecture?

Windows Media Center is a DirectX application, and in some cases leverages DirectX Exclusive Mode. You can't use DirectX objects directly from MCML.

>> Can I feed a Filter Graph into an MCML page and have text overlays?

It depends. To get video into <Video> you use the PlayMedia method from the Windows Media Center API.