That is precisely what I started doing, apparently reinventing Enum.find_value/3 as @ruslandoga pointed out 
Custom type is possible and what you suggest is an option, which crossed my mind too. But still tried to ask first and find out whether someone knows a way out w/o any custom type(s).
Keyword lists or just ordered key/value pairs are unknown to RDBMS engines as far as I am aware so even if you don’t code a custom Ecto.Type you’d still end up writing the same code anyway but someplace else.
So go for something that would be stored as JSON / JSONB in the DB in the form of %{v: keys_to_values, i: keys_to_indices} and have proper cast, dump and load functions and you’re golden. You might even open-source it in the form of an e.g. ecto_ordered_map library, if you feel generous. 
A very interesting solution.
Just curious: how about an embedded schema with fields :key, :value, :index and using embeds_many?
Then you put that into a map and order the list by the index and then you can further reduce it if needed, but you don’t lose searchability and filtering.
I feel I am missing something.
Don’t think I am following. Put what into a map, and what map? The OP asked about an ordered list of key/value pairs.