Underseal is formulated for use on metal, wood is a natural material and breathes. If you feel you really must put something on the underside use something like a microporous wood treatment which allows the wood to breathe. Sealing it could cause more harm than good.
Most chassis are galvanised so shouldn't require any further coatings, but if you want to go "belt and braces" I'd opt for something like Waxoil which will repel water, but "bleed" to repair itself if scratched or chipped.
Waxoil will also displace water, so it's fine to use on cables etc, but obviously keep,well away from brake drums and shoes!