Quick question, does signing players from my Japanese feeder club actually improve the amount of merchandising I get from them? How about doing a tour of Japan? Or any other methods to get the most out of my feeder club?

good question,
in real life im sure a tour and signing a big japanese player would make it better
but on football manager im not so sure

anyone know?

You'd think so otherwise what would be the point of having this feature in the game?

Not really a definitive answer though is it?

In the past, I know i've had actual messages to me in my news when i've signed east asian players, commenting on the increase in merchandising revenue.

So yes, signing players from there increases it.