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Trading soulstones for things the Imperium wants could make for an interesting story. A rogue trader/Inquisitor/governor/Magos has a deal to trade soulstones for STC's. It starts off with simple or small amounts like 50 soulstones for better farm tools etc. Eventually it turns into more soulstones for more important STCs like more durable boots or slightly better lasgun batteries. Finally, a huge deal for an immense amount of soulstones in exchange for something like the cure to marine-pattern baldness or a stable plasma gun is made and other parties ruin it one way or another. Something along the lines of Dank Eldar steal the soulstones and give them an STC that releases super flying aids or something. The less tolerant parts of the Inquisition/Admech seize control of all the other STCs because the Eldar clearly can't be trusted so they are never seen or heard of again. The Imperial person who arranged the deal is killed horribly by either the Imperium for heresy or the Eldar who just found out they gave a huge stash of soulstones to the wrong Eldar.

Although to answer your question, it probably varies greatly depending on how high ranking the person is and how involved with the Eldar they are. I imagine the Eldar are pretty secretive even by their standards over "these stones are the only thing stopping us from eternal painful torture in the afterlife so please don't mess with them." Deathwatch techmarines are probably going to have a few stories about playing with them. The Deathwatch RPG has an Iron Hands tech marine named Xerill who interacts with Aeldari and Necron technology extensively.