Douglas and all other interested in this problem.
Generally, in a genealogical database (like Geni is) it is a standard to use name at birth (both first- and lastname (lastname may be a patronym)) as the main name. All other names used later in life are secondary to the name at birth.
The namefield "Lastname at birth" (or whatever it is in your native language) is only used by Geni and no other software as far as I know, so in my opinion, as a genealogist, it should not be used. The person's first lastname should instead be written in the Lastname field.
All other names used later in life belongs to the "Also known as" field on Geni, since Geni so far doesn't have the possibility of Alternate names that most offline genealogical software have.
I know I will be hammered by this statement, but I'm only saying what the genealogical standard is saying about how names should be written in a genealogical database and stating the facts of how it works on Geni as of this time.