This is untested ground. Yes, the PPI belongs to the creditors, as confirmed by the courts. But what pressure the former IP can bring to bear, or is willing to bring to bear, has not yet been, as I say, tested.
During the IVA you had to comply with reasonable requests and authorising PPI claims was deemed reasonable. Non-compliance then could see the IVA breached .... but now ??? If an IP was willing to "go all the way", he / she could seek to have the IVA rescinded through the courts. I could see an IP willing to go to such trouble (and cost) if the PPI refund was enormous .... but would they do so for, potentially, a fiver ? The prize is unknown until the claim is made and assessed by the creditor.
I am afraid this is a "Brexit Question" .... nobody knows until it happens.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk
You may wish to speak with a specialist solicitor and get a 'set in stone' answer if it going to bother you too much. Personally I would just sign the forms and send them back, you are not going to see any of the money even if you make loud noises it will make no difference.
The definitive answer to your specific question is "no you do not have a duty to fill the PPI forms out". As Foggy says, it has never been tested, do you really need the hassle or stress of repeated letters from your ex IP to test the theory?
Last Payment made 04/12/14. Completion Certificate 25/7/15. IVA company GT. No Issues