The LPT port is a bit obsolete nowadays, but it indeed used to be a really easy way to interface to PC's. Unfortunately you can only draw a very small amount of current from the parallel port and the voltage will drop below 5 V when loaded. There may be computers with more powerful ports but you won't know until you try.
I'm not sure how much current a typical Canon camera will need to it's USB to trigger cable detect. Typically you can't draw more than a few milliamps from shorted LPT pins; enough to drive a LED but not enough to destroy one.
Combining current from several pins using schottky diodes might be worth a try, although I'd start worrying about port driver power dissipation as it probably wasn't designed for this purpose (i.e. you might fry your parallel port, be warned).
You can get slightly more current out of a RS-232 serial port (also kind of obsolete), but you'll need to limit the voltages as officially the levels are more than 5V.
But anyway, for either port... once you manage to toggle the voltage on any pin by software, you can of course use it to switch a slightly more powerful external power supply (like the voltage from your USB port or a lithium battery many of us use for remote shutters) to the USB port.
