Chach,
Make sure the marker reference is set to "the first placed drawing of the viewpoint." I've not had any trouble with the detail marker not referencing. I use the "built in detail marker." Also this way if you move something around it constantly updates, you should not have to manually input anything. There shouldn't be a work around...
On another note, I used to do enlarged floorplans as you currently do, blowing them off plans and having them turn into 2D lines and fills. From suggestions on this forum I was pushed to do it differently, and I am completely satisfied with the results. Simply go to the bathroom in plan, save it as a new view with a larger scale, and give it a new layer combo (I call my layer combo "enlarged plans"). I also have a layer called "enlarged plans."
This way the plan is always updated, you never have to worry about rebuilding the detail. In my office a lot of people messed it up where they'd do changes in the blown off detail plan when they should have been doing it in the main plan and updating down through. This eliminates those errors.
If you do it as I suggested just create a detail marker that is not a new detail but a "linked marker" and link it to the drawing on the sheet.