You could add in some lighter roasted Central?
That current Mex El Tru is a cracker - I've done some brilliant 2 bean blends with 50/50 Eth/Mex - Kisoro should work just as well too. The lighter pre-roast blended Mex (all that acidic citrus goodness) seems to really work well the the slightly darker choccy Africans.
Good luck with that!
Though who will complain about huge chocolate in their coffee anyway! Not this little black duck :-)