maybe you ought to consider using a film with better reciprocity charateristics than HP5. Tmax 100 is much better than HP5 and Acros is that much better than Tmax. Acros is linear out to 2min (really think about the implications of that!, where Tri-x begins to "depart" after 1/2 second.
Acros becomes faster than Tri-x at an indicated 2sec exposure. I often use Tri-x out to a 4 sec indicated exposure and switch to Acros for anything longer.
Thanks, Tom. I never looked into acros after giving up on TMX and moving back to traditional emulsions. But with those kind of reciprocity characteristics, I'll have to order a box and see how it performs.
On my original question, after reading almost everything available on the usual photo-chat sites, I've come to the conclusion that my notion of film becoming "desensitized" during extended exposures was pretty silly. It's a relief of sorts -- who needed another variable to battle?