Hi Joaquim,
You can contribute with respect to the develop branch. Everytime it is needed, I push it into the develop branch on boostorg (the official github repository).
Same thing goes with the master branch on uBLAS/ublas. I push it to the master branch on boostorg everytime we need to update it.
I can add you as a contributor to the project and you can create you own branch to do your experimentation. When you feel the code is reasy, we can push it to our own develop branch (and the boostorg develop branch at the same time).
Let me know if you have any question.
Best,
David