ok, check your current workflow here:
-click the shortcut for "make planar"
-click the type-in box
.. there are so many operations here, so how is it going to make you any more efficient to have a mode that will set all verts absolutely instead of moving them relatively? You need a key to enable that mode anyay, so you're right back where you started.
You're really barking up the wrong tree as far as improving your efficiency goes.
The most efficient approach here, if working the way you do, is to have two type-in boxes.. one for each mode. I've made a script fox a box like that, but not for this reason.
When inspecting this closer - why would you want to move all verts to 10cm or any other position? The most common answer would be because you want to align them to existing verts at that position. In that case what you need is a copy/paste vertex position script, not some fancy type-in mode. Such a script is a MUST-HAVE no matter what, and I'm sure you can find ready-made solutions on scriptspot, preferably one that lets you paste in both modes.