The question here is how DUN is added on in Maemo 5. If it's just BlueZ configuration then, depending on the restrictions about modifying (rather than adding) the BlueZ files on the signed rootfs, it might be more possible than if a recompiled BlueZ is required. And also what you mean by "at the same time" :-)
It depends what you mean by "out-of-the-box functionality". If the functionality touches the low levels of the system, the chances increase that it might not work with a more locked down runtime. However, Quake 3 isn't available out-of-the-box, and that kind of additional application should still be trivial to ship through maemo.org Extras.