Over in this topic we've been playing with using a custom SBF to replace the bootloader logo/image on the Droid 1 (bootloader image, not boot animation). A couple of folks have gotten adventurous and tried to do the Droid X. While the Droid X does indeed use the exact same scheme for it's CG42 (bootloader image), that doesn't mean you can use the same SBF. Hopefully if you tried it would just fail, but I could see it just as easily trashing the phone (the CDT between the two devices is wildly different).
So, the process of making the CG42 itself (helper) is the same, but the SBF is different.
Here are a couple SBF's to get you pointed in the right direction.
Note: Most of the code groups on the DX are signed and can't just be replaced at will. CG42 is not signed, so you can replace it without making the phone angry.
The original bootloader logo (package link in case you want to go back to stock at some point) looks like this:
I also made a custom one (package link) using the DF logo by Dandroid and Johndroid:
I would have also done TParker76's logo, but I figured someone might want to do that one "for homework".