If one compares pixel count, yes. But "pixel count" is not video resolution. Video resolution is measured using horizontal line/row count.
480 horizontal lines is standard definition video. Anything above this is high definition - for most camcorders (up to the Canon XF series), that means 720 or 1080 horizontal lines/rows. Yes, the T2i has an 18 megapixel imaging chip - but it does not use all the pixels when it is self-limited to support its highest quality 1920 x 1080 files. One difference I see between this and the HVR-Z7 is that the Z7 captures HDV format at 1440 x 1080. Most folks would not notice the difference. The other is that the T2i shoots 1080 "p"rogressive while the Z7 shoots 1080 "i"nterlaced. Again - at this level - most mortals won't be able to tell since both would be able to capture 24p or 30p.
And horizontal line count is only one measure. I fully expect the Z7's imaging chip array to out-perform the T2i under low-light conditions. This may not be a concern to you.
If the third camera is "reference sound only" (and video is not needed), why not jut use an audio field recorder like one from Fostex or a Zoom H4 (or Zoom Q3HD)?
I am currently shooting a television series on cable with a Sony HVRZ7U, editing on Vegas and outputting 1080i.
I am toying with the idea of adding an extra camera by purchasing the new Canon rebel eos T2l.
I shoot only reference sound on the third camera. It is mounted on a tripod.
Shouldn't the Rebel outperform the Sony for image resolution??