The Highlander was derived by blending together many different pieces of art from what I understand(as was the Flashman), but all of the art in question had to have showed up in canon sources.
Personally, I'd say the Highlander was inspired by many sources but is definitely an entirely new work of art. (Same as I would say for most canon artwork of established chassis from one artist to the next.) I immediately recognize it as a Highlander but the details are slightly different. The forward-swept armor of the cockpit is most notable to me, but the LRM launcher arrangement and left arm weapons stand out as well. Also, I don't recall a canon depiction of the -734 or -738, yet we have those variants sculpted. I can't claim to have every piece of Highlander art to reference, but I do have a lot.
I don't state this to be argumentative, I do so to highlight that there is a way for CGL and IWM to get things done if projected earnings make it worth doing. I was on the QCC for several years and we were bound by canon artwork, period. I would dig out as many sourcebook images of the units as I could find in order to give the sculptor as much flexibility as possible, but it was still very limiting. The current QCC is very good (IMO) and constraints appear to have loosened somewhat, which is a good thing. There has been one artist that I can recall (IAMCLANWOLF) willing to make isometric artwork for traditional sculptors to reference, and this new artwork apparently is approved by TPTB at CGL. Converting a 2D image to a 3D sculpt has always been tricky and if TPTB are willing to be a little flexible with the art in order to get an even better 3D product, we're all that much better off.