What I am suggesting to you is create a "drawing cell" instead of mechanical cell. Because mechanical cell requires height.
In the cell editor itself "drawing cell" option available as a subtab.
Select subtab and create new cell.
1. Create one new user layer in setup parameters.
2. In that layer add your notes whatever as your wish.
3. Save the cell.
By using library services you can move this cell to local library.
By using place part and cells---->Drawing cell you can place it into your layout(No symbols required to be placed in schematics).