right to a confidential recommendation letter

My former employer held a round of lay-offs recently and I was affected. I signed a severance agreement with them in order to receive some pay continuation and a percentage of my bonus. During the meeting with HR, it was made clear that the company was just cutting costs and that employee performance had not been a factor in that round of layoffs. After the layoff, my former manager approached me privately and told me that I had been a good employee and that he would be more than happy to provide letters of recommendation and references. Some time later, I approached him and asked him for a school recommendation letter. I explained that I would be signing the line on the form where I waived my right to see the letter, so that the recommendation would be completely confidential and I would never be able to see it. At that point, he told me that corporate HR had sent a memo to all department managers forbidding them from giving recommendations or references. From the context, it was obvious that the managers' jobs had been threatened were they to be caught providing recommendations. I spoke with corporate HR and explained that the recommendation would be completely confidential, but was told that the decision was final. They said that my former supervisor could write a letter of "personal" recommendation, but that he would not be permitted to make reference to any interraction we had while I was "on the job." Since our relationship was specifically job-related, this would make the recommendation useless.

I'm not asking for a recommendation on corporate letterhead from the company. I just want a recommendation from this specific person, who had originally offered to do so out of his own kindness.

What right does my former employer have to dictate what employees may write on their own time? Can they forbid him from discussing every single interaction the two of us had while he was acting as my manager? Do I have any recourse? Is there anything that I can do about this?