WRT the "deployable ELT", the C-5A had something called CDPIR, can't recall the words to the acronym, but it was mounted on the top of the T-tail and was either released by pilot action or deployed at some loading and would float. It was a maintenance nightmare, often failing its BIT test and requiring an avionics man on a high lift. In the end, it was removed on the B-models and replaced by a conventional ELT.
It's possible, but cost is the issue