I don't think the Varia has enough battery to last more than 3-4 hours on a single charge. However, I just tested mine out and it *will* operate while its charging. So your best bet is some sort of portable power supply. If it were me I'd probably use a compact power bank, stash it in my saddle bag and run a charging cable to the Varia. Something like this:
https://www.amazon.com/dp/B005X1Y7I2...ogi&th=1&psc=1
A generator hub might be another option but I've never used one and I don't know if they provide enough power to simultaneously charge and operate the device.