In reply to cha1n:
After talks with KLM they weren't able to allow anything exceeding their dimensions that weren't mentioned in their sports equipment section (without a huge oversize fee of 600 euros total).
Ended up booking flights with SAA from Heathrow (and a brief stop in Johanisberg) to Cape Town. SAA have bouldering mats listed as sports equipment (up to 200cm total for l+w+h), and one item of sports equipment is free in addition to a checked bag.
I'm going to take both my pads (one of which exceeds the 158cm dimensions by 20cm) and if they don't allow it then I'll stash it in my car in airport parking. Gotta be worth a try.