1) They won't work to South Africa as they do not work on partner airlines (other than UA).

2) To search for flights that can confirm at booking you should use the Advanced Search form and tick the box for OnePass Upgrades. You can also do a regular search and click on "fare class" in the right column of the results and look for the R bucket to be populated. That's where award upgrades come from. RN might work, too (it will in the future but the timing of that taking effect isn't really well known). If you use a greasemonkey-compatible browser the script here makes scanning the fare class data pretty easy.

Coordinating the upgrades and the award seat will likely be a challenge, but it is possible.