If you only want to filter on load, in the createscript you can use the RemoveLookup function. I think it's called that. It's been a while since I did scripting

Same here I've done this two ways, the 'RemoveLookup' option - which with workflow is straightforward as you don't even need to worry about doing IF / ELSE clauses. (this works well if you're just wanting to remove a handful of items, any more an it becomes a bit of a maintenance headache) 

Secondly I have created 'dummy' selection lists and then within the Workflow via script 'changed' the Lookup Family for a selection field so it suddenly references a completely different list (so one master with all values (which is the true Lookup Family) and then 'sub lists' which only have values that suit the different scenarios - whilst it needs a little work to maintain, it works well for big lists)

Same here I've done this two ways, the 'RemoveLookup' option - which with workflow is straightforward as you don't even need to worry about doing IF / ELSE clauses. (this works well if you're just wanting to remove a handful of items, any more an it becomes a bit of a maintenance headache) 

Secondly I have created 'dummy' selection lists and then within the Workflow via script 'changed' the Lookup Family for a selection field so it suddenly references a completely different list (so one master with all values (which is the true Lookup Family) and then 'sub lists' which only have values that suit the different scenarios - whilst it needs a little work to maintain, it works well for big lists)

Thanks for the ideas.