Is it possible to remove options from a dropdown post sourcing? I am trying to limit the options of subsidiaries users can select when creating a vendor bill after they source the entity, but based on the documentation "removeSelectOptions" is only usable for custpage. Is there some other option to perform this action? I read another potential solution where I display a custom field with limited options and populate the real subsidiary field based on their selection.

from a purely suitescript point of view I think everything you've said is accurate, and there is no simple way to do this and the workaround is a custom field you can control to act as a go-between

Yea that is what I was thinking at first and may be the easier method. Only a handful of people actually post anything (typically JEs) to the other subs so we probably can just remove their access, but allow cross-sub viewing. I was testing a simple workflow to return user error in addition to blocking records within the vendor bill approval workflow may be enough to catch the issue. I just didn't like the idea of the subsidiary dropdown being full of subsidiaries that can't use.