Well, I've never given a seconds thought to creating a custom plymouth theme, nore done any research on how one might go about that, with the exception of the past 3 minutes, which leads me to believe it wouldn't/shouldn't be all that difficult.
I had a look at the package contents of plymouth-theme-charge. All of it's files are in a single directory: /usr/share/plymouth/themes/charge
All the files but one are .png image files. The non-image file is a plain text file named charge.plymouth
BASH:~/-> cat /usr/share/plymouth/themes/charge/charge.plymouth
Description=A theme that features the shadowy hull of a Fedora logo charge up and and finally burst into into full form.
Seems pretty simple and almost self explanatory. It seems an obvious correlation between the "[two-step]" heading and the rpm package plymouth-plugin-two-step, which installs but a single file:
BASH:~/-> rpm -ql plymouth-plugin-two-step
two-step.so is a compiled binary, which is I'm sure the "engine" that runs the theme graphics display, and possibly other stuff. Hack away. I'll bet it wouldn't take too much searching though to find some documentation/tutorial on how to create a plymouth theme though.
is worth a read. Was hoping for some official documentation at the wiki site, but ney. Just putting in "create plymouth theme" in Google search brings up a slew of hits to pursue for more clues.
Edit 2: Four part plymouth theme guide linked to from the plymouth wiki site.