I have been following this thread for a long time. Today I read some post from another laptop-realted forum abut pin-modding ThinkPad R61E, which has a Celeron M 550 installed. It is a sinle-core Celeron M running at 2GHz (133MHz * 15x) over intel G965 chipsets. The author successfully overclocked it to FSB 667MHz and then 800MHz, putting the Celeron M 550 into 2.5GHz and 3GHz correspondingly. Another one followed and reported a successful overclock of his Celeron T1400 (dual core instead of single core) at FSB 667MHz.
I didn't check the details of the pin-modding, becaue I know they are all similar and also not-so-hard to manage. The point I am making here is, there is no lock-down to the lowest multiplier 6x when you pin-mod a low-end Celeron, single-core or otherwise. So the lock-down seems only to apply to mainstream or high-end Core branded mobile CPUs by intel's chipset (not sure about the Pentium branded mobile CPUs such as Pentium Dual Core). Also there is a link confirming this kind of checking and locking of over-clocked CPUs by the chipset: http://www.freepatentsonline.com/6535988.html
Following the above, an idea to get around the lock-down issue is to make the chipset think the CPU is not over-clocked, in other words, reporting a different reference clock signal to be as same as the over-clocked one. For example, say a T2050 running at 1.6GHz originally, being over-clocked up to FSB 667MHz, then upon bootup we need to fool the chipset and make it recognize the CPU has an original clock of 2.0GHz. To do this, there should be another set of pins to be modded (bent or shorted). Maybe it is time to go back to the datasheet and check if this is possible.