Hey there,
It seems like custom scripts placed in the repository custom scripts task directory are only showing up when right clicking over a single Task.
When right clicking with a list of Task Selected, the Custom Script disappears.
Bug or current limitation? It is an easier way to assign custom task scripts to all tasks with right click than going in to the job properties and setting it globally for the job.
You need to enable multi select for the script.
We would need some help on this topic as the link above is inaccessible with error 403. So can you please let us know how to enable multi-select for the custom script? Although the one we are using is a copy of an existing Factory Deadline script with a few added python variables and re-formatted output info. Yet the custom script is not showing in the multi-selected job’s context menu! However, the same is working fine for a single job selection.
Thanks,
Bhavik
Fractal Picture
Ah! nevermind, I figured it 
Tools → Configure Script Menus → Job Scripts → Edit Selection → Enable Multi-Select