Google for a program to do something, and find an old reference to something to do it :-)

The follow up post then says an updated and working version of the code is at another URL, but that's broken by now - bah !

So I emailed the poster, and overnight he writes back with the updated code, which nearly works. A quick bit of hackery later, and here is the latest version of the perl code to add a datasource (ds) to a round robin database archive (rrd) as used my rrdtool etc.:
http://rachaelandtom.info/falken/add_ds.txt