Mine has been propped open for years, but... it leaks vacuum and I have maybe 15 seconds to exit the car and lock the doors before the vacuum bleeds down and the doors won't lock.
So I found a good dashpot on a 300D in the U Pull yard, someone had already pulled the dash so I got a good look at it. Not only did the MBZ engineers make it very hard to get at, but they also installed it with one-way single use spring washers on plastic mount studs that cannot be removed without destroying the studs. So I may order a new one anyway.
Here is a link to a thread which discusses how to replace the center vacuum dashpot. PeachPartsWiki: Replacing the HVAC Vacuum Actuators
If you had a borescope, an assistant, small hands, and a lot of patience, you just might be able to do it this way.

Good luck!