How do I then write to a heap descriptor? HBufC is not derived from a modifiable descriptor class?

First you must call HBufC::Des() to get a TPtr, which is modifiable. You can then use that:

TPtr fredPtr(heapbasedFred->Des());
_LIT(KDes,"My Desc");
fredPtr.Copy(KDes);

further reading http://descriptors.blogspot.com/