For what it's worth, what I've seen in other games boils down to "if you have to ask, you probably shouldn't do it." IE, if the game includes tools and instructions for modding something, it's fine to mod it. If it doesn't, you probably can't without breaking the EULA.
first off has anyone got a reliable tool for extracting the hogg (pigg) files?
I haven't bothered to roll one suitable for random people to use, but it's not even remotely difficult. The hogg format is an uncomplicated archive, with a directory in the usual style. The compression algorithm is good old zlib (off the top of my head, I can't remember whether it's 1950 or 1952).
However, if you are only going to _look_ at the files on _your_ hard drive, then you are free to do as you wish, it is, after all, your computer.
If you want to modify them then you need to comply with the license agreements (i.e. this isn't allowed).
There is nothing to stop you from writing a tool that reads data off your hard drive - remember, this is how the Hot Coffee was found in GTA...
If the tool turns out to be awesome, then please get Cryptics blessing and post it up on the STO intel wiki