I think it is likely that Hen-Boisen might originally have been Den Boisen or any of the other variants on "Boisen" that you gave. There is a town in the Netherlands called 's-Hertogenbosch (also known as "den Bosch" which basically means "the woods".) Of course, it is common for a surname to be derived from where the family lived. And since, as you said, your family is from South Africa, home of the Afrikaaners who are of Dutch descent, I think Hen Boisen is an Afrikaaner variant (I am not familiar with Afrikaans, this is just a guess) of Den Bosch. Don't take my word for it, but it might be a lead to investigate.
Orlando, Florida, USA