Funny how things turn out to be (sound familiar..?).
Again, focusing on something totally different, the obvious was missed. Monodevelop of course has T4 support, because much of .NET development starts to rely on it.
Just smoketested the PowerPoint Add-In abstraction on Mac OSX and with acceptable adjustments (file path separator and minor systematic tweaks) the abstraction generated the full add-in.
I will be pushing the “Hello World” abstraction asap to GitHub for Monodevelopers to try out.