Is there a usage for  as "rèn"?
No.
「」 has no usage examples as the reading rèn (Bopomofo: ㄖㄣˋ). Note that many ancient dictionaries make definitions up out of thin air, which is why quotes from character usage in literature is so important.

At least one dictionary author, smelling something fishy was going on, said


《字彙補》
...疑「」字之譌

Suspected to be a corruption of 「」 (rěn).

So it might come from 「」 with a horizontal line added to the bottom.

Further reading
See the reference given by Wiktionary: Dictionary of Chinese Character Variants (教育部異體字字典), A03215-002