I think to pull off portrait keyboards (based on analysis of a few binaries and on looking at the source code for ukeyboard and ukbdcreator) we would need:
Cloned header files for libimlayouts.so.0 (library used to read the keyboard layout files)
Complete clone of libhildon-im-vkbrenderer.so.3 and vkbrenderer.so (the rendering logic for the virtual keyboard)
Probably clones of other libraries.
A complete new set of portrait keyboard layouts that properly encode the keys so that the code knows which row the keys go on (the layout files contain row information in their data)

So in short its going to be very difficult to pull off.

EDIT: looks like vkbrenderer.so is a symlink to libhildon-im-vkbrenderer.so.3. Doesnt make reverse engineering it any easier though