Dynamically filtering a CTE?

I have a slow ‘WITH RECURSIVE’ CTE like:

CREATE VIEW ancestors AS
WITH RECURSIVE _ancestors(descendant, ancestors) AS (
SELECT
item.id AS id,
ARRAY[item.ancestor_id] AS ancestors
FROM items AS item
UNION ALL
SELECT
child.id AS id
child.ancestors || ancestor.ancestor_id AS ancestors
FROM _ancestors AS child
JOIN items as ancestor
ON child.ancestors[array_length(child.ancestors, 1)] = ancestor.id
)
SELECT *
FROM _ancestors
WHERE child.ancestors[array_length(child.ancestors, 1)] IS NULL;

I'll usually only need a few rows, so I'm being bitten by PostgreSQL's
CTE optimization fence [1]. I'm looking to optimize it by limiting
the dynamically limiting the number of rows in the initial query, as
if it had been:

WITH RECURSIVE _ancestors(id, ancestors) AS (
SELECT
item.id AS id,
ARRAY[item.ancestor_id] AS ancestors
FROM items AS item
WHERE {your condition here}
UNION ALL
…
)

My initial thought was to create a function which accepted a WITH
clause as an argument. Something like:

CREATE OR REPLACE FUNCTION ancestors(condition)
RETURNS TABLE(id integer, ancestors integer[]) AS
$$
WITH RECURSIVE _ancestors(id, ancestors) AS (
SELECT
item.id AS id,
ARRAY[item.ancestor_id] AS ancestors
FROM items AS item
WHERE condition
UNION ALL
…
)
…
$$ LANGUAGE SQL;

or with ‘WHERE condition(item)’. But I couldn't find a way to define
an argument that was a where condition [2] or a record→boolean
function [3]. I could probably use PREPARE/EXECUTE [4] to dynamically
construct the WHERE statement, but that looks like it may have its own
optimization issues and there's no way to stash it for use in
subsequent sessions. Perhaps a function to run the PREPARE? Is there
an idiomatic way to approach this problem?

Thanks,
Trevor

[1]: https://www.postgresql.org/message-id/201209191305.44674.db@kavod.com
[2]: https://www.postgresql.org/docs/10/static/sql-select.html#SQL-WHERE
[3]: https://www.postgresql.org/docs/10/static/sql-createfunction.html
[4]: https://www.postgresql.org/docs/10/static/sql-prepare.html

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