I put my custom dialer on n/p along with a few other things via root explorer. When it said that I click the button at the top of the screen to toggle read/write mode. If that isnt working for you. You are one of the few getting root access issues from installing froyo. Check your superuser permission on Root explorer if thats what your using to push/pull files. Im no help if your using pc to do it since I found that method was more trouble than its worth.
EDIT: anyone use automount with froyo yet. I cant even install it. First app I have that refuses to install period on it ;(
I hate having to manually mount everytime I connect to pc. there has to be an easier way on froyo.
EDIT 2: just got an update for audiomanager I hope this fixes my issue.
So easy to pop things onto sdcard then use root explorer to push them to system/app.