You may know already that there are several references to WILEMAN on the NAAIRS index
www.national.archives.gov.za/If the family came into Natal by sea, it's possible that the surname may come up on the European Immigration Index and I will check that for you when next I am working in the EI index in Pietermaritzburg.
As to the origin of WILEMAN, it may have a similar derivation to WILES or WYLES - from Old English meaning 'trick' or Middle English 'wyle' which was a basket or trap for catching fish. Another possibility is that WILEMAN is a contraction of WILDMAN, which meant just that.

Regards, Mole