Ah just tested that in a map document. It opens an editting session in memory, but does not actually show the editting session within the map document itself. I can edit the attribute table manually of the feature but can't manually edit geometry. I thought the map document would recognize the edit session, but apparently not.
Interesting, I can't even edit in the attribute table.
I didn't write out a whole script tool to use, I just entered all the values in ArcMap's python window.
Also interesting is when I opened and closed an editing session within the map document manually, it closed out my in memory editting session. Apparently their aren't many pythonic ways to get a map document to check if there is an edit session ongoing. Found a few other threads that could point you in the right direction, but I'm not sure using arcpy.da.Editor is going to do the trick by itself.
If anyone else has some suggestions, I'm all ears.