Do you want to randomly rotate only to certain items, or what is the reason to not use Transform->Random ?
In this case, you can do this:
1) Create a separate Forest object with only the selected items, using Tree Editor->Detach Selection.
2) In the new Forest, apply Transform->Rotation.
3) Select the items in Custom Edit mode, and click Tree Editor->Store Transform (the transformations are stored within each item, and Random->Transform is disabled).
4) Merge the two Forests, using Tree Editor->Attach Single.