Oops. A mobile substrate plug-in is basically a 'program'. It's binary format, not like a text or plist file thats easily modifiable.
Update:
I looked 'inside' 20secLockscreen.dylib and it appears to modify the system variable "restartDimTimer". I also did a search of the iPhone filesystem and did not find this variable in any standard modifiable text/plist file. This implies it can only be modified by changing 20secLocscreen.dylib (or another program). As I said before, because it's binary -- I don't think it's easily changed and don't feel comfortable attempting to change binary files.
I also looked at the last source for Supreme Preferences. It add/changes the dim time-out when the phone is unlocked, but I don't think it changes the timer when the phone is in a locked state -- at least I couldn't find one

If it really does, then it's modifying a different variable that I'm unfamiliar with.
I think we're stuck unless someone wants to code something.
I welcome additional comments from our resident experts





