Based upon the previous-generation 27" iMac models, the max was 32GB (Using 4 x 8GB 204-pin), and I have not heard of any 16GB dimms, so currently my answer would be no. That being said, I am not an expert on this, and I may be incorrect.
The max RAM support from Intel base on the CPU release date. Not account for any new higher density RAM. I will call it "max demonstrated", rather than "absolute max".
e.g. My W3690 suppose only support up to 24G of RAM, and I am currently running 32G of RAM in my old Mac Pro with this CPU, and lots of other guys running 48G, someone even push it to 56G. It's more than double of the max from Intel.
Of course, no one can guarantee that this 5k iMac can upgrade to 64G in the future (when the 16G stick available), but the chance is there.