Have found the answer to my question, and am posting it in case the question comes up again for other FTers.
A "Marco Polo seat" is simply a seat held for Marco Polo members. Usually these are good seats (like those juicy ones upstairs) or seats close to the front.
45DEFG is not even bulkhead, but the row behind it; it is considered useful for Marco Polo members since it is close to the front of the coach section.
However, I was unable to discern anything useful to me about it, so I changed to the more open row 66, 67, 68 double seats in the back of the plane.
BTW, this all started when Cathay called me about the CX883 cancellation. I think they were trying to soften the rescheduling blow by offering "Marco Polo seats"...
I'm now looking at a full redeye flight where the best choice is to sit near the toilets. This vacation is off to a flying start!