I was able to mod the save with the hex editor a bit:
I was able to change - How much slots for stuf you have, so isntead of the starting 12 --> 50 for example, 45 is amximum normaly, and it still works ONLINE ^^
Then I was able to change skillpoints to 200, level to MORE than maximum (50 is max, changed it to 255 and it works, more can crash the game) and finaly the sniper weaponskill and amount of greandes you can wear. Haven't found more yet. I just used it on a new fun char, I don't want a max level or higher char^^. I just tried out what's possible. But I don't know how to create a savegame editor or mark a line which always changes the value.
So I hope someone can do it^^. It looks like it's pretty much the same from the xbox360 savegames. That's all I can tell.
That's all I can do for now, hope someone can find out more and find a way how to make a tool out of it
Here is the savegameeditor for xbox360, I don't know if it helps, but maybe it does
www.modcontrol.com/Board/xbox-360-savegames-cheatingtools/20944-borderlands-save-editor-v1-2-1-a.html
PLEASE do NOT play online with hacked inventory. It corrupts other players' save files. There's no reason to hack your inventory anyway unless you just like being a pack rat with your old guns, and even then, you can create "mule" characters to carry them for you, and hack THEIR inventory so they don't corrupt anyone else.
It's probably also likely that hacking ANYTHING beyond the in-game limit corrupts other peoples' files, so only play your hacked character offline. If you're going to hack an online character, do so in moderation, don't push anything over the limits so that you don't screw anyone over. Don't ruin legitimate players' save games and online experience.