Hi Scott, coming from Scotland I get jobs like this quite a lot. After all it is the nations second favourite drink!

To be honest it'll depend on the type of carpet. On a polyprop Ive managed to fully remove using fibre & fabric rinse with no other stain removers..... then you can get a cream 100% wool in which case the cases are slim of removing it fully. It'll also depend on the size of the stain and how long it has been there.
Before you do anything pre-qualify the customer that the stain may not come out.
Red-Rx is the product that I prefer. I always start with just a rinse to see if I can remove anything, then next stage is to use the red-rx with the damp towel and hot iron. (Please just remember to pre-test this before you use it. When I first started up I got some offcuts of carpet to practice on. Tried the red-rx on a yellowish wool carpet and it took the colour right out

)
Always finish off by rinsing thoroughly with an acidic rinse, especially on a wool.
I know this might be what you are doing already but as u know Irn Bru is a killer to get out.
Good luck
John