Nope, not sure why you would want to either.
Last phone I had to overclock was my android HTC Hero that had to be overclocked to run well. I hear the common thing to do on android is to overclock them to try to remove the lag or generally improve performance over time. There isn't the same need on WP phones.
My 920 runs much better and more stable than any phone I've had with no lag at all. So, yeah... Everything is locked down and I'm not aware of any way to do it. WP 7/8 phones have a reputation of running really quick because WP is optimised well. Rest assured your phone is quick today and will be just as quick in a year or two pretty much.