While messing around with the ME on the Slim, I found something interesting. Once you turn on the ME, you can no longer change the CPU frequency. You can change it all you like before it's turned on, but afterwards, changing the frequency hangs the PSP. I've tried turning off the ME, but I'm not sure I have a nid right for 3.71. Could someone verify sceSysregMeBusClockDisable is 0x07881A0B on 3.71?
Using that, and sceSysregMeResetEnable, I've seemingly stopped and restarted the ME, but I still can't change the CPU frequency.