Answer to your first question: there is no officially stated reason. Sources at Canon once told me that the 4GB clip length limit is there to prevent standard definition recordings from exceeding 30 minutes, thereby avoiding a significant tax penalty upon buyers in the European market. There's also the possibility that the 4GB clip length limit is self-imposed in order to protect sales of Canon's professional video camcorders series (XF, XH and XL H models). However, there are a number of other ways in which a D-SLR can't replace a pro video camera, so that reason isn't a very strong one.

Answer to your second question: you don't. The outgoing HDMI signal contains graphic overlay information which cannot be removed. Long-form recording of uninterrupted clips is more suitably achieved with a proper video camcorder, such as the afore-mentioned XF, XH and XL H series cameras.

Is either point changeable by firmware update? Unknown, but not at all likely to happen. Perhaps a third party will undertake it, but not the manufacturer. Hope this helps,