R – Perforce – submit only files open for branch, not files open for edit

I do not think that this is possible when issuing a "reopen for edit". As you have already indicated, the reopen command will change a files status to something different. I ran a local test (I'm running 2008.2). There do seem to be some changes in the way that perforce reports on the add/edit status of files since your version, but what you are trying to do still doesn't seem possible

d:\sandbox\ctg_test>p4 integ test.txt test_branch.txt
//ctg_test/test_branch.txt#1 - branch/sync from //ctg_test/Test.txt#1,#15

d:\sandbox\ctg_test>p4 opened
//ctg_test/test_branch.txt#1 - branch default change (text)

Note that the opened command now shows 'branch' for the file status instead of add which I think it might have shown with your version of the server. But the kicker:

d:\sandbox\ctg_test>p4 edit test_branch.txt
//ctg_test/test_branch.txt#1 - reopened for add

d:\sandbox\ctg_test>p4 opened
//ctg_test/test_branch.txt#1 - add default change (text)

I tried looking with the -ztag option to see if more info was given:

d:\sandbox\ctg_test>p4 -ztag opened
... depotFile //ctg_test/test_branch.txt
... clientFile //client-mark.allender/ctg_test/test_branch.txt
... rev 1
... action add
... change default
... type text
... user mark.allender
... client client-mark.allender

p4 fstat was no help either.

Sadly, you're on the money here, you'd need to use a third party diff tool to do this because, as you rightly pointed out, until you've submitted the initial integration to the depot, it doesn't have a copy against which to diff. If you go down this route you'd obviously not be able to access files directly in the depot though, you'd need to have both files on the client PC and specify their paths explicity.