I don't think there is any crank arm other than one made by FSA that will fit that crank. You're saying, "Well, wait, why can I get a different crank arm to from Stages or 4iii to fit my crank if it was a Shimano crank?" Easy, the crank arm you buy from those companies are Shimano crank arms, with the pwoermeter bits installed to it. If, or when, Stages does the same thing for your FSA crank, then you could also buy a FSA crank arm. That being said, I am not highly familiar with FSA cranks, but, If I am not mistaken they have a couple fo different specifications for how they assemble their cranks, so you need to be sure if you do find an arm based PM, that the arm you can purchase is compatible with your crank....someone familiar with FSA could answer those questions if/when a arm based solution is presented.
Okay, Quarq DZero Dub...funny you ask about this. I just put one on my SL7 about a month ago. Actually, I put on a DFour, a DZero that accepts 4-bolt 110BCD Shimano Dura Ace 9100 rings (and Ultegra 8000, though the spider is visually matched for the DA9100). That spider (DZero or DFour) ONLY mates to a DUB crank. Therefore, if you were to go that route, you will need a new crank, and a new Bottom Bracket (since DUB is a BB standard). DUB uses a different diameter crank spindle than your current crank.
Let me ask this, and I am not trying to be a jerk, if you are so intent on having a PM why are you opposed to using a clipless pedal based solution - which will work immediately without any kind of crank (and possibly BB) replacement (which is the only way you're get to a PM with flat pedals)?