I've used it. It will fit just barely, max diameter is listed as 67mm and that's what the RF100mm has. But that shouldn't be your main question, as getting a step-down ring to use with it is really simple.
I'd recommend NiSi close-up attachment instead though. They have two very nice +5 diopter ones (different sizes), and just came out with a +9 diopter (stronger than Raynox). You will need adapters to use either of these, but that shouldn't stop you. I own and use both versions and absolutely love them. I have a macro-dedicated camera and one of the attachments stays on pretty much all the time. I sometimes unscrew it when I find a larger insect.
I found two issues with Raynox that made me switch to NiSi, and there's a potential third one.
1) Raynox has high distortion around the edges, it's very noticeable and if you try to produce non-cropped images the quality will suffer. No such issue with NiSi - it's cyrstal clear all the way out to the edges.
2) On smaller lenses (this might not be an issue with the RF100) it's very easy to misalign the attachment resulting in more distortion. This is because the attachment is held in place by two springs and there's enough friction in the "buttons" to allow misalignment.
3) Some people have reported losing the attachment as again, it's just held by spring tension on 2 small tabs pressing against the filter threads.
The ease of attaching Raynox does not compensate for its shortcomings. A lot of macro photographers do use it, but I think that's just because they don't know better. NiSi is really a hidden gem. If you do go this route, I would recommend buying
directly from them. Their customer service is phenomenal.
