I was just successful in having this honoured. Will be traveling to South Africa next week and as we've seen, many of the HIs down there have been rebranded as Southern Suns...the name of the company which operates ICH properties throughout much of this end of the continent. Unfortunately this means many properties no longer participate in the PC program.
However, one that still does is the HI Katherine Sandton. I booked two nights there on the PC site at a rate of about ZAR1050 per night. Then when booking a couple of other properties in other cities on the Southern Sun site, for some reason this same HI popped up, so I checked their rate for the same nights I had just booked on the PC site. Low and behold, the Southern Sun site quoted ZAR850 a night for the very same category room! So I decided to see what would happen if I filed a "Best Rate" request by email and within a couple of hours received a message back acknowledging the rate difference and advising that the hotel would be advised to charge me the lower rate, less 10%. A nice savings that will translate into a few Windhoeks at the hotel pub, plus breakfasts...
So it does work in some cases...but just remember to make the claim within 24-hours of having made your PC booking.
I suggest anyone planning stays in South Africa, or other countries where Southern Sun operates HIs and ICs, to book on the PC site, then check the Southern Sun site for the same property and room category. It's sure to be cheaper and then go make the claim with PC. (I think this might also work in Australia/New Zealand where there is a similar corporate umbrella that markets ICHs and its own brands -- Centra, Park Royal et al -- and thus has a separate bookings website frequented by locals.)