What about the really rich people? You can only hold 1 bil zeny on a character, so I guess that would be the max limit. We have some people on the server who are richer than that.
Also, it would require a eathena source modification, which means that its harder/takes more time to code and test than just a normal NPC script. Is it really worth that effort for such a minor problem? I'd rather see the GMs spend their time adding some fun and useful stuff.
the banker, don't take him away from me.
What I do is just keep most of the zeny on one char, and just enough to unlock warps, use the healer, etc. on the others. Then if I need to do any heavy transactions I go on that one character. Its not much of a bother really.
My chars: TPC / Tjuven / Dansare / Sinne / Magiker / Byracka / Krigare / Andreas