This is probably the best solution we can get. I like it because it links the two main warships with transports, and enhances their value on amphibious, but without coopting the specific ability of the transport itself.
The only thing to account for is how Battleships and Cruisers interact with transports that are unloading 1 Inf + 1 Artillery, or 1 inf + 1 tank.
For simplicity it is probably best to treat all land units equally. The only concern might be the potency of the existing artillery unit when combined with this new form of battleship and cruiser support.
I might suggest, if it proves overpowered when combined with normal bombardment, that this new land combat role should just supercede and replace normal bombardment. This would simplify things and lead to less confusion overall. Especially since bombardment as it stands is rather complex (the whole casualties firing back aspect, and separate combat phase etc.) The idea suggested above would eliminate that weirdness and streamline everything into regular combat, just with the bonus added. Seems fairly easy