Cheese & garlic sourdough baguette?

Although I haven't tried this, I read where bakers steep garlic in the same water they will use in their bread formula. This way the garlic flavor permeates the entire loaf. As to the cheese... probably doesn't steep very well :-)

I see much experimentation in your future.

If you put cheese, butter and garlic in the centre of the baguette when shaping, you will end up with a large empty hole down the middle of your baguette with a bit of oily, garlicky residue in it. :)

I suggest you mix or fold the cheese and garlic into the dough and see how it turns out. Or make focaccia and put the oil, garlic and cheese (and a bit of rosemary) on top.

I've always used Hamelman's cheese bread recipe in Bread as a kind of rule of thumb and haven't gone wrong yet. 

I add at most 20%, no matter what kind of cheese, after mixing the rest of the dough to a moderate level of gluten development. Then proceed with bulk ferment etc. as normal. I find it easier to mix in small and very cold pieces of cheese. This avoids the dough instantly feeling "greasy" in the hand, and also avoids big holes in your crumb from the chunks of cheese melting during the bake. The cold cheese will reduce the dough temperature, though, so to counteract this I use a proofer for the bulk ferment. I'd just add the garlic at the same time although I am not sure of the percentage to add. I'd be very wary. 2-5%? 

You can also just roll your dough out flat when you come to shape it, sprinkle it with cheese, sundried tomatoes, etc. then roll it up tight. Slice off inch wide portions, then put them on their side for the proof (sort of like a pain au raisin). I often do this with odd bits of dough that are too small for putting in my baskets.