No. USB streaming means low quality video and you can't get a clean HDMI feed from a T2i either. Tethering yourself to a laptop instead of buying a couple of memory cards is as absurd as it is impractical.
*** NERD ALERT, NERD ALERT, NERD ALERT ***
Actually, technically it is possible to do this and the short answer is 'yes, theoretically' but I've never seen anyone dumb enough to do it.
Assuming you have the right port on your camera and you could somehow take the analog signal before it hit the internal digital convertor or purely from an analogue AV port you could output analog signals through an external digital convertor and if you connect the camcorder to an A/D convertor, and the A/D convertor to the computer's USB port you will end up with a low quality video stream being pumped directly into your Mac. This is tricky but you're persistent so you might get this far.
If you get this far, this would provide you with an extremely low quality but live video stream.
Now the tricky bit is to find a piece of software which will let you monitor the garbage coming into your machine. All the usual FCP, iMovie etc... on Macs won't let you monitor this because no-one is dumb enough to do anything like this but technically although it's a video stream. It'd be useless for stuff like colours, contrast and all the things you need for filming but technically, you'd have video. However, to actually be able to see it, your best bet would be to hire Chinese developers to create your own piece of software or alternatively you might get lucky and there could be software out there that would do this for you. Of course, even if you were to do this, the image quality would be so bad as to be useless and the Mac would have to be uber high end to be able to process the information live.
However, on a pure technical basis, this is a solution. So what do I win?
But instead of asking the question relating to a USB cable, if you changed the question to 'can I use a Mac as a monitor with a live visual feed?' the answer is 'yes.' For example, the 27 inch iMac should be possible. However, this depends on the output signal and how this is converted.
What I mean is you can use an iMac as an external monitor but this requires a Mini DisplayPort to Mini DisplayPort cable. It would not work with a HDMI cable or a DVI cable, even when they are connected, for example, to a pair of Apple Mini DisplayPort adapters.
From a technical perspective, the iMac 27 will work only with an mDP output signal. So unless your device outputs mDP (which it doesn't) you will need a converter/scaler to use, for example, an iMac 27 as a monitor. You'll have to search these out on the 'net in relation to your particular piece of kit, check how it can convert from your particular camera. Oh, and don't forget to put the iMac in Target mode (reboot and push the "T" key) and you have to use a displayport connection. I like to be thorough.
Oh and in case I forgot to mention buying a 27 inch iMac, the relevant kit and sorting it out will cost a fortune not including the ridiculous amount of time you'd need to spend hooking up all these bits of kit and getting them to work.
Note that most of the guys at the Apple Store won't know this but if you really want to spend all your money on trying this out, go onto an uber-tekky Apple forum and seek out the nerdiest, geekiest looking avatar (generally something displaying someone as an overmuscled superhero-type), they'll be able to direct you.
Aside from this, there is a cheaper and better solution. The one which readily comes to mind is BUYING A FRIGGIN MONITOR.
Next week's lesson - do I make a car myself starting with creating my own mine to get the ore out or should I just buy one ready made? Hmmh, tricky one...
