There's a caveat with charging an electric toothbrush using your phone, though: Your phone might not detect that something is actually being charged and so to save battery, it'll automatically turn off reverse wireless charging.
So unfortunately I don't know if you can actually fully charge an electric toothbrush this way. Even if you could, you'd need to constantly turn reverse wireless charging back on, which is a huge pain.
H/T to FragmentedChicken for the idea, though. If this properly worked, I'd definitely bring my electric toothbrush on trips!