Thanks Gerard and everyone else who responded. Setting par.label := <some
variable> in a field guard linked to the caption was the answer. I made a
mistake somewhere the first time.

The forms editor allows inserting of a) a set of standard controls, b)
variable text (see above) and c) pictures. Is it difficult to enhance or
extend the forms editor to insert any view  which is under control of a
program? It's the same principle. That way instead of a specific set of
options just about anything becomes possible (a moving graph, an animation,
3D). 
Another very simple enhancement could be the inserting of a few pre-defined
standard elements like lines and boxes to layout the form more clearly.

This would not just spruce-up the forms editor. It makes view programming a
lot easier by just programming the component views and letting the 'forms'
editor do the assembly. In turn it would make the whole potential of
BlackBox much more visible.

Perhaps most significant is that it could make BlackBox a lot more fun!
Quickly creating colourful & 'dynamic' forms could be just the right
advertisement.  

Erik