At the moment, there's no ability that I can see to customize the toolbar in Resolve's Fusion tab. I hope that's something that will come later. In standalone Fusion, I have a few scripts on my toolbar. Resolve also doesn't allow us to create custom items in the menu bar. I doubt that ability will come back, but I certainly wouldn't be upset if it did!
Failing a customizable toolbar, you do have other options. For instance, I have some scripts that I run from buttons in certain nodes (again in the standalone, but it should work in Resolve as well). My Savers have one that creates a Loader linked to the rendered file. Very handy for prerenders. If you add a User Control (right-click the node and choose Edit Controls), then save the new configuration as a default (right-click, Settings > Save Default), every time you add that tool to the Flow it will have the User Control in it. There's a thread at Steakunderwater about running scripts from a Button control:
https://www.steakunderwater.com/wesuckl ... 60&p=14333You could also make a floating script panel using the UI Manager API. I'm not sure if it's possible to make it pop up upon loading the Fusion tab or not, but you definitely can set a hotkey to run a script, so you could easily invoke the panel that way.