The RAD on the H100 is 27mm thick. If the RAD can fit under the lid (which it can), then 25mm fans certainly can. The problem actually is fitting the RAD and fans INSIDE the case. I don't think it's possible, without hitting your RAM and/or motherboard plug-ins.
Here are some nice pics that show the RAD under the cover and fans inside the case. Not a lot of room.
If you look at one of the pictures I linked, the standard 25mm fans are attached to the top of the case, and that's about the height of the RAD. It's a bit hard to tell how much space is left from the angle, but to me, it looks extremely close to the RAM sticks and the top power connector. In the end, I guess it will boil down to the motherboard, but I'm not so sure even 12mm fans would work here.
The 500R case top reservoir has about 29mm of clearance, so sticking even 12mm fans on top of the RAD up there won't work either, unless the OP is willing to leave the top mesh cover off.
I was able to do it using the 12mm fans rad in case and supplied corsair fans on top pulling. As another poster mentioned its a pretty tight fit but if you take your time and are patient its not impossible.