I'm writing a mod to essentially make any map have a buyable ending. I have everything in a few custom named gsc files, but buyable_ending.gsc contains all the calls to the other code. I want to insert

map\buyable_ending::init()

Into any map (hopefully) regardless if it's a Treyarch built-in or a custom map. I thought I was clever in using _zombiemode_utility::init() since it was empty, I figured it contained anything needed from the previous map styles and is typically left unchanged in custom maps.
I add the buyable ending call in the zombiemode_utility init and it works fine for Der Riese. When I try, say, Shi No Numa, it fails looking for a _zombiemode_net (or something like that). This would likely mean I would have to include a bunch of other scripts to get it to work. I want to keep it as clean as possible and prevent including a script that a custom mapper would change for their map and risk breaking things.
Anybody have an idea on how to do this? I'm sure I'm overlooking something simple ...

If anyone is curious, I found this worked best. All the stock maps are fine and several custom maps with this as an add-on worked.

_callbackglobal::SetupCallbacks()