There's a program called RegJump, by Mark Russinovich, that does just what you want. It'll launch regedit and move it to the key you want from the command line.
RegJump uses (or at least used to) use the same regedit window on each invoke, so if you want multiple regedit sessions open, you'll still have to do things the old fashioned way for all but the one RegJump has adopted. A minor caveat, but one to keep note of, anyway.
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