If you are using version 25, then switch to zone labels instead of stamps.
You can add as many labels as you like, in any layer, at any scale.
You can even label zones in sections.
The zone stamp will have its origin where the user moved it. If you draw symmetrically to the origin, it will be anchored to the middle.
Text is drawn with "paper size" height, but its coordinates are in model space. For scale-independency, you need to transform to paper size.
text2 0,0, "O"
mul2 GLOB_SCALE / 1000, GLOB_SCALE / 1000 ! 1 m model is now 1 mm on paper scale-independently
add2 -5, -5 ! move origin of drawing