Thank you, Meandering Mouse. I'll be sure to drop in from time to time.
Renevr: Can't answer for SANParks, but can offer a botanists-eye view of the problem. Unfortunately, scientific knowledge of trees changes (improves?) over time, and so names change, and more plants are found that just scrape it on to the list of trees. As a result, although or maybe because the list you remember has been maintained over the years, it's now a mess (not to put too fine a point on it), and really needs to be scrapped and started again. Not only because we now have to insert names between names that were inserted between names (etc.) with consecutive numbers on the list (what was 147.17 again?), but also because the systematic ideas underlying the sequence of the original list are now known to be, er, suspect. So yes it would be possible to put numbers on trees again (if one could think of a foolproof way of doing so without damaging the tree), but with all due respect to those who want to hang on to the list we have, imho this would really not be a good idea until some fairly basic revision of the list has been done.
That said, there is some good news. I see there is a thread here on DNA bar-coding the KNP trees. One of my research projects involves making an electronic key to 'all the trees in Africa' -- using a list that the bar-coders and I at least talk to each other about occasionally (I don't have formal links with them, through nobody's fault at all). It would be the easiest thing in the world to add numbers to this list when we're reasonably sure it's decently accurate, and probably not far off the second-easiest thing to publish it once we've worked through the issues of what common names, in what languages, what about different concepts of the same species yadda yadda yadda. The obvious advantage of doing so wuld be that if you see a tag numberes 12345 (it could hardly be less than 5 digits) on, say a Cape Chestnut in Addo / Zuurberg, and you recognise the tree, then when you go to Kenya and see another tree labelled 12345, you'll be able to greet an old friend.
Thank you for bearing with this long lecture.
Time flies like an arrow, fruit flies like a banana. (Groucho Marx)