Hm... Though wouldn't you have to take the seeds out and put them back in every time you took the belt on/off? Assuming your character does more than just plant seeds (ok, there are some characters which are only used for farming but I think that's a very small %) it seems like that would be pretty inconvenient...
Just dedicate a box/chest to seed storage near your fields and problem solved

Aside from the issues with the mechanics, I do like the idea of a seed bag belt though... Imagining a guy waddling around with 20 seed bags wrapped around his waist
