Is there some special technique required to insert a class cube on a custom palette? I've tried several times but it doesn't seem to be working.

The Class Cube is a built-in function and I don't think you can access it. In the palette editor, when selecting "Display path to palette file...", you get a message saying it's built-in.
However, if you create a control and put the class cube in (type def or not), you can insert this wrapper control in any palette or save it to user.lib.

That particular cube is for "LabVIEW Object", which is indeed built-in. But for classes in general, you should be able to add to the palette just by selecting the .lvclass file. However, this is not working in LabVIEW 8.2, 8.5 or 8.6 (it did work in LV8.2 beta, but broke for some reason in the actual release). No one noticed that until very recently. But the workaround, as has already been mentioned, is easy enough.
If you want to insert the block diagram constant into the Functions palette, you can create a VI that has nothing except the constant on its diagram and then add it to the palette as a Merge VI.

Along these same lines, how come in my OOP code, my class data is a cube, but on some other code I have inherited, it's a cluster showing the class data? How do I get that later thing?




I think you are referring to Place VI Contents. What used to be called Merge is now the deal when you're sopposed to be able to merge two VI's, which is great demo at shows.

There is no cluster view of LabVIEW classes -- the data is private and there is no way to get access to it (including FP access) other than through interfaces the programmer exposes (FP access can be exposed through an XControl if the programmer chooses).

You may have inherited code written with one of the GOOP toolkits, perhaps?

I bet you're right The originator of the code was a hardcore C++ programmer. It wouldn't surprise me if he used 3rd party OO. I have a good idea... I'll ask him. Sounds radical, I know.