Daniel,
Is there a mechanism that I have not kept up with?
I just talked to one of the Asterisk developers and he wasn't aware of anything else, though that doesn't mean there isn't one.
The purpose of dynamic feature codes is to give the ability to implement 'in call' feature codes similar to the stock ones like transfer, attended transfer, automon (which FreePBX stopped using in favor of our own dynamic feature code refered to above), etc. It's also designed to provide a level of security by defining at call time if it's accessible.
Can you elaborate or provide a link on the capability you're referring that will allow you to store the DTMF dialed and execute it during the call. (I suspect he wants to stay on the call with the person to make sure the door gets unlocked so it probably has to happen during the call?) Since there are often many different ways to solve the same problem in Asterisk, maybe I'm missing something here?