Hello Gentlemen, while I am no expert on the subject I too have been doing a lot of research on uniforms for the Rhodesian forces during the Bush Wars and have become friends with two former Rhodesian soldiers. One, a former officer in the RAR and the other a junior enlisted soldier in the RR. . First, what figures/scale are you using? I ask this because, depending on the manufacturer and scale, you can get pretty specific figures for what you want to game. I do 28mm using Eureka figures and though less extensive than some of the 20mm figures available, you can still cover just about everyone.
All that said, even if the figures have the "trademark" RLI shorts, you can use them as basic Rhodesia Regiment figures as well. I have a copy of Alexandre Binda's, The Rhodesia Regiment (RR) and there are plenty of pictures showing RR soldiers in the field in shorts, though they tended to wear the boonie style head gear.As the war drew to a close, the uniforms had started to become more regulated though you still saw troopers modifying there uniform and kit in the field.
For uniform references, I highly recommend Alexandre Binda's other two books, The Saints (on the RLI) and Masodja (on the RAR). Both are a wealth of info, have great pictures, many/most in color, AAR's from battle contacts that are great for scenarios and they even have a DVD in the back that shows their history. The Saints is hard to come by and is expensive if you find it. Masodja is less expensive but just a valuable as The Saints. The other book, The Rhodesia Regiment was printed by another publisher and lacks a DVD and color pictures.
Anyway, I hope this info helps.