I would have thought both platforms would be the same... (goes off and tests)... however, they are not.

I have just now tested the script to fetch a locked post from Dreamwidth, and the same locked post made by the same person (for whom I have the same level of access on their DW and their LJ journals), both using the same exported cookie file which contains the appropriate session cookies... and it succeeds for Dreamwidth, but fails for Livejournal.
Fails in the sense that Dreamwidth gives me the page I want, and Livejournal gives me a "please log in" page.

8-(