ScriptMaster and the POI library
Not trivial, but do-able
We figured if it were doable it would be quite an undertaking. Since my post we've steered the customer to generating PDFs from the data instead.
Thanks for the reply anyway, John!
Spent a couple of hours playing with it today. We use the library extensively for creating xlsx files, and it was a useful exercise.
Easier, and with better results, if you go with .ppt rather than .pptx as the XML part of the library is somewhat less developed - as in, wouldn't suggest it for production but that kind of task could be done