This got me wondering, are they capable of such a feat in a motherboard that doesn't support EIST (SpeedStep)?

For Prescott-based Mobile Pentium 4 CPUs, I see it as a possibility as their SpeedStep functionality is software-based. According to the datasheet, they specifically utilize MSRs to switch clock speeds.

For Northwood-based Mobile Pentium 4 and Mobile Pentium 4-M CPUs, I see no possibility as their SpeedStep functionality is hardware-based. Again, according to the datasheet, they utilize a single pin (GHI#) to switch clock speeds.

Realistically, I believe the Prescotts might be better off in this case as their version of SpeedStep uses MSRs. That being said, has anyone ever tried using a desktop PGA478 motherboard to run a mobile Prescott at its maximum speed? Did it run at its maximum clock multiplier? If it did not, could you try these set of steps to check if it works? Thanks.

ENABLING SPEEDSTEP, THE ODD WAY:
1) Download Read & Write Everything v1.7 from this link: http://rweverything.com/downloads/RwPortableV1.7.zip (requires XP SP3 x86 or 2000 SP4 with blackwingcat's extended kernel)

2) Extract it out from the zip, and place it anywhere that you prefer.

3) Run Rw.exe, and then click on the icon that looks like a flipped PGA478 Pentium 4. It has a text labeled as 'MSR' on it.

4) A window with the name of 'CPU MSR Registers' should pop-up, once that loads, click on the 'User' tab.

5) Click on the 'User' icon/button, and then copy the text below to its text box (not the text box found on the left side of the 'Add' button). Click on the 'Done' button once you have entered them in.
IA32_PERF_CTL=0x199
IA32_MISC_ENABLE=0x1A0

6) Double-click on the newly created register called 'IA32_MISC_ENABLE', now you should be able to see 64 bits shown in a small pop-up window. Do not click on any of the registers yet.

7) Double-click on the number below register 16, it should be then flipped to '1'. This forces SpeedStep to be enabled, but be aware that this may cause a BSoD. Make sure that you have nothing important running at this moment. Once you have done that, click on 'Done'.

8) If it doesn't trigger a BSoD, congratulations. You have just enabled SpeedStep, assuming register 16 didn't reset itself back to '0'.

My last request if you have made it this far: did you notice anything in the IA32_PERF_CTL register? Are there any numbers in it? If there are any, what are they?