When I have needed to cut open a wallwart nicely, I do
it on a table
saw. This way I can control the depth of the cut, as well as having a nice
even seam. The only problem (and not much of one, really) is the gap that
might result from the wider width blades.
Waht about using a slitting saw in a milling machine (or lathe with
vertical slide)? I've seen those down to a few thou' cut width, and you'd
have excellent control off the depth of cut.
-tony