My first try for using GRI went like this:
squash two commits, edit message, click ok
resolve the conflict
Continue Rebase hangs up
Show Full Output:
git -c diff.mnemonicprefix=false -c core.quotepath=false -c "sequence.editor="/Applications/SourceTree (beta 1.6).app/Contents/Resources/stree_gri"" rebase --continue
Vim: Warning: Output is not to a terminal
Vim: Warning: Input is not from a terminal

Clicked "Cancel" and it dumps a bunch of garbage which looks like it might be a Vim screen:

...blah [0m[22;1H[1m[7m[1] .git/COMMIT_EDITMSG [gitcommit] 1,1-1 All[1;5H[24;1H[0mVim: Caught deadly signal TERM
Vim: Finished.

Turns out I had specified vim as my editor in ~/.gitconfig.
Once I removed "editor = vim", the GRI with an edited message worked correctly!
This setting has been in mt .gitconfig for a long time, and this is the first time I've had trouble, so I'm guessing this is 1.6b1 specific, but maybe not.