Hi,
We do not generally recommend LDOs to supply the REF pin since it does not have the ability to sink current. It may work, but the only way to know for sure is to test it. The AD7691 is proven to work well with ADR435, it's the recommended reference as per datasheet and was also used on the AD7691 datasheet. It can source/sink current up to 10mA only.
Regards,
Karen