I have a bicycle frame pack, I bought many years ago, that is made for a level topped main frame tube, when road and mountain bikes were made that way. It fits in the main triangle and is attached by three snap loops on the top of the bag to the top tube on the bicycle, where it hangs and stays put. Two rear loops attach to the seat post, and one on the forward downtube. Those three keep it from shifting forward and swaying. The whole thing can be removed in seconds.
I used if for years, bicycle touring, but have recently been thinking of putting it on an old Schwinn frame with the idea to use it for backpacking on local hike-bike trails, (not riding it), just to see if my 'hike-bike' idea works out.
What I'm considering is this: Strip the bike, including the one piece crank out of my Schwinn, Continental frame, a bike I found in an irrigation ditch. It would interfere with walking. Remember, this bicycle is going to be used exclusively to carry the weight of the backpacking gear, not me. I would also cut down some bars to a very narrow size and use only a front brake and a rear rack. I can put my middle bag in the main frame with all the heavy items and a large top bag on the rear rack, thus keeping things narrow and light so they won't interfere with hiking.
I would retain the seatpost and seat. The reason is to use the strait-arm-pull method of hiking with the bike, up hill. It's also nice to rest, leaning on the seat. On a level surface, I only need to steer with my one hand on the stem/handlebar and walk with the bike. On steep downhills, I can use the front brake, if needed.
This weird hike-bike idea has been swimming around in my brain for years. It would keep me on the trail as my damaged back gets worse and when I get too old to carry things. I haven't needed it yet but it couldn't hurt to give it a try and prepare for the future.
Anyway, hope the middle pack idea works out for you. I think it's a good idea, keeping the center of gravity low instead of on your back. I think the main concern will be to keep the bag from shifting forward and back and side to side.
Best of luck, Whirlpool.