I don't think that it would be reliable as the solution for several reasons:
a) Usually magento dev team upgrade their mysql db tables almost in every release (without proper documentation) so you will be forced to upgrade manually your db structure after every change (it's not wise to ignore magento upgrades, since they fix pretty big list of bugs everytime (and introduce some new bugs long the way :P)).
b) Magento's db structure is very complex, and it would be an hellish job to port everything in another type of DBMS with it's own reasonings for external keys and such
c) And in the last: I'm not sure that their db engine would support a new kind of DBMS (it should be Zend_Db but who knows).
I'd suggest you to convince client in using mysql (since you would need dedicated server anyway for running smoothly magento)