Well, rooibos isn't actually tea. Where tea leaves will become bitter if they're boiled, rooibos can probably stand a minute or two of boiling without throwing off bitter flavors.
Still, if it were me, I'd use the sam regimen that I use to infuse coffee flavors.
After flame-out, let the temp drop to the 205-210*F range and then add your coffee/rooibos to steep. This approximated the temperature at which you would ordinarily brew coffee or rooibos at. If I were using black tea, closer to 200-205 and real delicate stuff like green tea as low as 175-180*F.
Steep for five minutes for coffee, probably closer to 3.5-4 minutes for rooibos.
Then chill the wort down and pitch as normal.
I also like to add whole coffee beans to either the secondary, keg or in the bottles themselves. Puts a little cold coffee flavor in there as well after a few weeks.
With rooibos, I'd make sure to do that in either mesh strainer ball or a satchel of some sort just so it doesn't get messy inside the keg.