I guess it depends how confident you are with batteries.
I have a small 24v solar setup that I use to charge my laptops, phone and stuff but it would work the same for a portable battery. You can get 7S BMS's (battery management systems) and could make a nice portable system.
Batteries7S BMSDC to DC converterI tried 4S but found 7S worked out better for solar but either way you want the DC to DC converter just to keep 12v's going to the keyboard.
Then to charge them you'd just need a power supply that can output ~30v.
I currently use an inverter for my S970 but I'm ordering a DC-DC convertor so I don't have to mess with stepping up the voltage only to drop it again.
There's also plenty of cheaper options
Like thisAlthough they depend on the battery which aren't usually that good. At least with DIY you know whats going in it.
but don't seem willing even to take an electric extension through for powering keyboards etc.....(maybe a bit over-cautious ? although understandable).
Sadly common sense is heading to extinction which covid seems to have made worse. Plugging in an extension doesn't suddenly mean you're going to catch it. If you're doing an event, you're the risk not the extension! A breeze is more likely to transmit it between people.
So yes, you do what you can to minimise the risk like wiping down the plug/cables but if your that worried why even have a party!
I read a story yesterday where some are now linking it to Bill Gates wanting to secretly chip everyone! Just...