Under the File Menu/Form Properties in the Defaults Tab you can set a target version. If you have added features to the form that are beyond this target version you will see warnings in the Warnings/Report palette. To activate this palette click on Report under the Window menu.
I am going from memory here so the story may not be 100% correct but you will get the idea.
The UI to add the comb to the form was added in Designer 8. The xfa grammar (the part that describes the comb in the xml template) has been available from the beginning. So the interpretation of the grammar to the comb is OK. Previous to version 8 of Designer you had to code the xml by hand (hence the term it is not supported). Make sure you test it but I am sure that it will behave the way you want.
Hmm...interesting. Yes, it created with PDF as artwork underneath...but the target version is currently set to 6.0.2 or later. It was after I'd set the version to 8 that it wouldn't then let me set it to an earlier version.
And it seems to work ok in Reader 7.
I just tried a test setting the target version to 8.1 - as long as I don't close and reopen the file I can change the target version back. But if I close and open the file then I can't change back from 8.1 to anything else.