Fran, in response to your statement " Not sure what you mean by: "if I reassemble the UV mapped parts and then convert to a mesh," I will explain. When one creates a model from several booleaned shapes and then groups them it is a type of assembly. In the case of Bryce When a negative boolean intersects a positive one and is grouped with it the negative one cuts out the area of the positive that it occupies. That is one type of assembly that is possible. You can also group neutral shapes and even do intersections of shapes with the grouping feature. 

Now once a series of shapes are grouped, a small icon box with a C in it appears. According to the Bryce 7 Artist's Guide, clicking on this icon will convert the grouped shapes to a mesh; and indeed if you enter one of the edit menus, you will find an option in the list that says "Convert to Mesh" or something similar. Some people call it collapse; however, I have found that when this feature is used, the converted item become a single object that cannot be disassembled. It can be booleaned and grouped with other objects just like the primitives. This can then in turn be converted as well. In this way some really complex models can be created.

So what I am actually saying is that after I create the model and save the separate parts to my objects library, I would export them to a UV Mapping capable program, do the UV Mapping, save the parts as .obj, import them back into Bryce, reassemble the model with the UV Mapped surfaces applied, and finally click the C icon to convert the model to a mesh. If you prefer the word collapse instead, then that's what I mean. 

Of course the description of what I am explaining is a bit complicated, but I cannot think of a simpler way to explain it. Hopefully you can understand it.

Stay awesome!

Agent 0013.