Just pondering this.
A lot of real-life criminal organisations ostensibly start as defenders of some group of poor people. Or so they like to claim. The Yakuza claim this and so does the Cosa Nostra. The Somali pirates referenced earlier claim they started as fishermen being cheated of their living by foreign boats fishing in their waters, not to mention illegal waste dumping and suchlike.
So. What if a capsuleer is from a largely poor populace, or has ties to same? The pirate group starts with the declared intent that it is defending it's baseliners from a capsuleer threat of some kind. Now I think of it, the antagonists in Clear Skies 3 come to mind. If you pick an area of FW low-sec you could easily rage against the contending militias on this basis alone.
This has its ups and downs. The problem is, as has already been referenced in the thread, that capsuleers can by no means be considered poor, so all options along that line don't suit. Then again it can be, as you imply, defending other poor folks from basically capsuleer exploitation. There are a few other ideological points where a similar thing can be done.
I'll bring up two examples.
Intaki separatism of the early type, which is what I used as part of my own exploits in the early years of Eve (along with a "pro-continuation war with the Gallente" philosopghy for the Caldari who joined up). The unique situation, with Placid as low sec, gave that possibility and later elaborations on it have established that this is in no small part due to Intaki demands which made it all the more sweeter (coincidence that it turned out that way after me starting it up as basically an excuse to pirate? think not).
The second obvious area is Minmatar resistance fighters, who start out fighting the Amarr, blame the Republic for basically surrendering in the face of the enemy, and refusing to stop all out war against the Empire. Now, without Republic support, all-out-war is really out of the question and boom, very soon the ideology loosens and piracy sets hold. I guess Ushra'Khan kinda did this, but they never allowed themselves to plunge into proper piracy.
But you could do it with almost any ideology. The more oomph it has the better, though, because it resonates with a larger part of the player base. You could rebel against a local corp strip mining your belts, but it is less epic (though it might be exactly what you want). The Minmatar thing is spelled out clearly in PF. At the time, the Intaki thing wasn't, but I felt it was implied or at least that the empty space around what was known fit. One could go for something a little more obscure, too.
I think key is keeping the ideological hold pretty tenuous (which includes allowing its influence in setting objectives, choosing enemies, making GalNet threads). It is replaced by the "ideological" components of piracy. Former freedom fighters/terrorists become independent freebooters spitting in the face of all authority, feeling betrayed and abandoned by their original ideologies. The old ideology lies there in the background, submerged, giving depth and complexity, but it isn't the driving force. Otherwise, they are gonna say "you are not a pirate!" (they'll say that anyway) and you will feel a sting in your heart. Your heart must be cold, black and unfeeling. Piracy and the life-style that choice implies is what's what.
So, here's the formula:
1) Take an ideology or cause
2) Enforcers of said ideology
3) Betrayal or abandonment by their own
4) Choosing piracy out of necessity and spite
5) Embracing the freedom of the pirate lifestyle
That worked for me and if I were to do it again... I'd prolly go the Minnie route myself.