Wangu is pronounced with the "g" after the "n". And I'm not sure why you would want to spell it without the pronunciation of the "g" since the correct way to pronounce it is with the "g".
I'm just talking about the spelling in general, not this word. ie How to write the velar nasal (/ŋ/) sound alone . I was using a theoretical word "Wang'u" as an example.
you are right about the ng' having a nasal sound in Swahili (it would sound like the "ng" in the English word "singer")... wangu doesn't fall in that category though i.e. no apostrophe after the "g" so it's pronounced hard as in the English word "hunger"