Okay, I just got the chance to take pictures of my system. It's currently in a Mini-Box M350, I had the case laying around and I felt like it would be a good temporary fitment test.
My conclusion from using a 120mm fan on top for reference, like you suggested, is that an adapter like yours that is centered over the fan mounting points should work. It will be a close fit against the CPU power connector, and it's possible that the fan casing/frame would be flush against the connector, but I think the wires could still be routed out and around.
I'd have to invest in low profile RAM like you have but that's something I'd be happy to do for the result!