Letting yourself be summoned for boss fights is a quick way to collect lots of souls. Usually I've been returning to Majula as soon as I have enough souls to level up, but today I've been summoned several times without pausing. As a result I collected 200k+ souls. At this point it seemed like I was being targeted more than my compatriots in the boss fights, even though I was certainly not the one dealing the most damage, and often by a boss I had ignored in favour of another one. Is this a placebo, or is there some mechanic which makes enemies more likely to target players with more souls?
No, they do not.
The AI in the game do not focus on anything other than distance, and damage. The closer you are, the more they will focus on you, unless someone is dealing more damage, that outweighs their "distance" aggro.
Having any number of Souls, or your Soul Memory play no part in an enemies targeting calculations. The only thing that can affect this is the Redeye Ring which increases your "Aggro multiplier", making enemies aware of you over a greater distance, as well as reducing your "damage requirement".