Why add all the extra weight, more things to catch on, no additional strength of most of the yellow tube?
Search NWHBC cage or Hybrid cage with my user name and you will find the original write up on the cage that "Those Guys" builds. You might also find some information on the fenders with the same search with the user name ODXJ? Jeremy did the original tube fenders that "those guys" have also now done.
There are also a tone of threads on other cages if you are looking for other ideas.
Michael