I'd forgotten about mask-renders (they existed in Bryce 3 - I think..). That makes things a lot easier:-)

I looked at the tutorial spiritwolf2000 so kindly provided, but got a little confused.
After rendering the tree (as image and mask) and deleting it, the tutorial-writer suggests making a terrain, using the mask-render to define the terrain, flattening the terrain (with a bit of clipping), applying the colour version as a text-map, rotating the terrain so it's upright, exporting as .obj, and using that...

Is this really more efficient than just using a suitably-sized (2-dimensional) square and applying the mask in the trans channel and the image in the diffuse channel? I only ask, 'cos the latter is the way I'd intended to go about it. The tut-writer may have hit upon something that has other advantages than my simplistic approach (but I don't quite see what those advantages might be..)

(That's always assuming that my B5 doesn't choke on the Abarro Willow - which I haven't got around to trying yet...)

Cheers,
Diolma