I would also like some guidance on this, before I get too far down the wrong path if I'm not doing it correctly.
Now, assuming that hispanic name customs are similar enough to how it's done in Brazil, here's an example of how I handled it for some Brazilian profiles I've created.
Milton Gomes Costa's Family Tree
So, for Last Name At Birth, I used "Gomes Costa", which is made up of the paternal part and maternal part. Now this might not be the best example since his father Oscar seems to not have followed the rule, but the paternal part, Costa, did get passed to Milton.
The profile for Milton's mother, Leonila, shows how I handled the name change due to marriage, so last name at birth in this case is "Azevedo Gomes" and the current last name became "Gomes Costa".
To me, this seems like the best fit for the available name fields. Anyone else have a different idea on how to handle this?