The ps3 is not a psp, period.
No you could not, ever since 2.10 you can no longer use infectus to downgrade even if you possess a nand dump from your own ps3 (not to mention that the new ps3s have /dev_flash stored on the hdd), beside even if you could, can you imagine the hassle it would be to do that all the time ?
Yes you could but then what would be the point ? There would be more efficient ways of running backups then.
Aren't you forgetting something ? The bluray disc binaries aren't signed to run from the hard disc and even if they were, the paths still point to dev_bdvd
beside the loading speed wouldn't be that better considering most games sport hdd installs, so that excuse holds on psp but certainly not on ps3.
A log is worth more than a thousand words (see below), but perhaps you could just install something on 2.80+ and decrypt the harddrive, then you will figure it out. And of course there are not any kind of public documentation regarding this point. It is not like sony is handling a ps3 wiki.