I'm having a hard time finding a mint buttercream that doesn't use extract so, I was thinking of making IMBC and, adding fresh mint to the sugar/water so it infuses while it cooks. Then, straining it and continuing with my IMBC as usually!

Any on try this or, have any experience with it to know how it will work?

Have you tried looking for peppermint oil? You can find that with other candy flavorings. Oh wait, I think Wilton has a candy flavoring set that includes peppermint.

http://www.wilton.com/store/site/product.cfm?id=3E3103E7-475A-BAC0-56CDA3C3D9B7A669&fid=59E629EA-475A-BAC0-5F72CC119638B946

Honestly not sure about your infusing idea. That's pretty thick sugar syrup and I think your mint would disintegrate during the cooking process (unless you tied it in a cheesecloth or something). I also think your BC could get a green tint to it. You would also have to use a lot of mint to get the flavor you want. But you could always try it on a half batch, and if it doesn't work well then use oil and mix the two together.