I literally just bought a cheap 7 port usb hub off ebay and then wired it to be powered externally by a powerbank (I use a 10000 mah 2.4 amp anker that works more than sufficiently for my purposes, and assuming your midi devices arent drawing crazy amounts of power that should be good for you too)
I got the idea from external harddrives that have y splitter cables
If you desolder the 5v pin from the usb cable on the usb hub, it will no longer be powered by your computers usb port, but can still communicate with it (keep the gnd connected!). Then you can either solder on a port or a wire(I used a wire since i dont have many micro USB ports on hand which I would have preferred, and connect it to a power bank. Woila! Rechargeable battery powered usb hub in 5 minutes
Note: some powerbanks have auto shutoffs if not enough current is drawn. My power bank uses leds and the lights were enough to keep it on without any issues.
I also forgot, i wanted to put a switch in so I could still the hub to connect (fewer devices) but still use my laptops usb power, so I dont already need the power bank.
I'll bring it to dorkbot tomorrow and do that there if you want to see it in person! Otherwise heres a simple instructions that you can modify if you want to use a powerbank like me
I couldnt confirm this but based on my raspberry pi usage in the past, should be safe to use most powerbanks as my assumption is that they are regulated power supplies. Anyone else with any insights id be glad to hear if theres an unforeseen potential danger here but worst case scenario is you dont get quite enough power to all of your devices
Best of luck
Athan