Filtering a Look up column on page load.

I'm trying to filter a look up column using something like CAML to only show me a subset of the values in a list. Basically when the New or Edit Form loads I'd like to parse a value out of the URL (in my case the Document Set Name) and then filter a the look up. 

To understand what I'm trying to do: The business requirement is to create a look up column to the same library we're working in that only shows documents within the current Document Set (very similar to a folder if your not familiar) as Options. 

If anyone can think of an easier way to accomplish this or something similar please let me know. Otherwise if anyone has experience dynamically filtering look up columns, I'm curious to hear how you accomplished it. 

Thanks.