We have serious problems getting next to black powder up here, but no problem getting the subs. So a hunting but worked with 777 quite a bit to see what it would take to make it work. He was able to get good performance only one way- He dropped a 10 grain charge of Goex 3f down the bore before adding the 777, then used the same Goex 3f for prime. While I have Goex 4f on hand and he tried it, he disliked it in the bore under the 777 for the main charge due to it's tendency to blow out the vent while seating a ball, occasionally resulting in slow ignition or failure to ignite the main charge, IIRC. In our wet climate 3f has actually proven to be a more reliable and trouble free primer than 3f, so that kept him from having to use three powders.
I haven't tried it myself, but since he just moved and left me with six jugs of 777 3f, I will probably get around to it someday.
Is it worth all the trouble if you have ready access to black powder? Not on your life. Is it something to keep in mind against a day when you can't get the black? Absolutely. I'll switch to subs and keep shooting, rather than giving up in a pout when I can't get black. I could stretch a remnant pound of precious black a long ways that way.
BTW- He also tried using 777 3f for prime and main charge too, with no black whatsoever. In an hour of trying I think he got one bang.