Jack,
When I last looked into 20g 'low pressure' loads, I found just a couple or few in my book "Reloading for Shotgunners" (Fackler & McPherson). But, it did come up here on the bbs and I acquired some data from another member. I don't recall the source nor did I have occasion for using the data. However, there are at least a couple low pressure loads in the book I referred to above.