The "Archives Year Book for S A History" 1951/II included an MA Thesis (University of Pretoria) by Melt van Niekerk : "Adolf Schiel en die Duitse Kommando". It is in Afrikaans, which will limit its usefulness to you, but the list of 694 names in Appendix III should be of interest.
Van Niekerk stressed that it was not complete, and in the more than 60 years that passed many new names must have come to light. It contains names of other nationalities, including Boers, that served in the German Commando
You can also consult the Scripta Africana publication "The Foreign Volunteers" by Brian Pottinger.