A sad update...
After yet another day lost on this insanity, I am giving up...
I managed to isolate the issue to the fact that its related to the fact that
one of the items my postflight moves has a resource fork. Sure enough, I
read a bit and learn that cp on 10.3 doesn't do them (an is throwing the
package exception that I'm seeing?). Fine, I say, I'll install CpMac to
/tmp and use *that* to move my .bundle (that has the resource) to where it
needs to go. Even more outrageously, when I now install it under 10.3
sometimes it errors out, and sometimes it's successful (my guess is that
CpMac is erroring out in certain situations when the dest file already
exists--I don't know what situations--its not documented).
So, basically, after two days of zero productivity and no answers, it's time
to ditch PackageMaker and go with a readme containing install instructions.
It's a little disappointing that 10.3 can't suit a lowly developer like me
who just wants to move a couple files around during an install, but here I
are. More specifically, I wish I could understand why 10.3 wasn't patched
so that it'd be on par with any installer made for 10.4. The whole reason I
got in the crisis that I'm in now (where all my 10.3 users are sending me
angry emails) is that I assumed a simple package that worked under 10.4
would work under 10.3. Sigh.