The Forerunners would not have wanted to honor Mendicant in any way, so why would they build a tomb or write an epitaph for him? Instead, I think that Epitaph could be a monument for Offensive Bias who they created to combat Mendicant Bias and Gravemind. They constructed the building that Epitaph is at the top of to honor/store Offensive Bias which would have been seen as a savior of sorts. This may have already been thought of, but all I see is "Oh, Epitaph is Mendicant Bias's tomb! Let's go find the tomb everyone!" so I thought this would be fresh.
-
When OB was made the last thing on the Forerunner's priority list was creating a memorial for an AI. It was towards the end of their war with the Flood and they were preparing to activate the Array. Their defences were being destroyed by the Gravemind and his new found AI buddy and they were becoming desperate and panicky. As someone else said in the thread where you originally posted this, I believe that is where OB stored the fragment of MB he captured during the battle. I also think that it is the location where MB was reunited with his lost half and also where he spent the remainder of his life.