I ordered the o2d2 kit for my DA62 at Oshkosh this year. I took a picture of the oxygen port in the 62 and showed it to the Mountain High sales rep. I got the 750 connector so that must be correct. You will also need a pressure reducing regulator. I also got the boom cannula that connects to your headset. They usually offer a discount for Oshkosh and probably Sun n Fun as well.
I talked to them over email and they mentioned the 750. I'm assuming they have it. I'll place my order over email or phone to make sure I don't get anything I don't need.
I verified with Mountain High today. If you own a Mountain High O2D2 and cannulas, this is the kit to buy to use the built in oxygen system in a DA42/62. It's the 750 connector to the O2 outlet in the plane, a first stage regulator and a little plumbing.