I tried to post here the other day, but Tek-Tips went down once again. -Sigh- I do have a variation to note though.
We had this exact same problem last week. The old webmaster/Lotus Designer had been gone for a year when we discovered a low-maintenance database to which only he had manager rights and to which we suddenly needed them.
Like you, we used his user.id, but with a variation. He left under bad terms so we could not contact him. We've kept copies of all the original user.id files that Domino Administrator creates and stores for us, even when the user leaves, which is perfect for this scenario. We've also maintained a hidden sheet which logs all the original passwords, so we indexed it to find his original ID. All that was left was to recertify the ID file with Domino Administrator before using the ID to sign in and change the database ACL.