An update:

Tried this very same thing in FontLab 4.6 in the Metrics Window
and it only imports the left sidebearings. How amusing.

Is there any way to import sidebearings from one font to another?
(Maybe there's an obscure Windows tool ... it would almost make me
go PC.)

Your logic saved me, Eric. This is actually a method I've tried
before on another project, but it didn't work then, so I didn't
even try it this time. But, whaddya know, it likes me now. Thanks!

Well shoot. Now I want to copy the kerns using that method and
I remember that's what didn't work when I tried before. Some
people on the FontLab list say this works, but not for me. The
sidebearings copy, but not the kerns. Do the fonts have to have
the same encoding? I am not sure how to change mine properly.

Ok...

I've copied sidebearings by using Paste Special and kerning by
importing kerning via "Open Metrics" in the Metrics Window.

Whew. Hopefully you won't have to hear my whinin' for a while.

Coles, going PC?
You must really be in a pinch.

Here's what you do (in Fontlab)
1.) Open both files.
2.) Select the characters from
from the file whose sidebearings you want to copy.
3.) Then Edit > Copy
4.) Switch to the open file to which you'd like
to apply the sidebearings and select the same
characters (this is key).
5.) Then Edit > Paste Special
6.) Select the left and right sidebearing options
and hit ok.

This should do the trick.
There are others ways to do this but this
is the fastest and smoothest.