Teaching Him to Tell Us He Needs Out

We just adopted an older dog (probably 11) from a shelter. He's a pit mix, basically healthy, and was in various shelters for the last three years. He is clearly housebroken and understands that he should be going outside -- which he does when the doggie door is open. However, if he needs to go when the doggie door is locked (e.g., at night), he'll simply go in front of the locked doggie door. He was no concept that he should be telling us that he needs to go out. I would appreciate any suggestions for teaching him to tell us when he needs to go. And no, I don't want to put him in a cage. He's spent most of his last three years in a cage.

If you don't want to crate him, which when unsupervised is for his safety, then just put puppy pads in front of the dog door when it is locked.
   
Grab one of his turds from outside and when he lifts his leg to pee, stick a jar underneath him to catch it.
   
Dump both on the pad. He will recognize the smell of his own messes and that will be a strong indicator for him that he is to go there.
   
Then hope that he realizes he is only supposed to soil there when the door is locked. Obviously remove the pad when the door is open to give him the idea that no pad means outside potty.
   
At his age he is unlikely to be trained to call your attention to the fact that he needs to potty but does not have access to his potty area so your best bet is easier cleanup of his soilings.