I am just starting to plan another DONE to requalify for AA EXP. The last time I did a RTW, paper tickets were common so there was no problem with 20 segments. CX computer prepared the ticket in Japan and I had no subsequent routing changes, so there were no difficulties. I plan to have CX issue the ticket in Japan again.

Currently are there any problems with 20 segments?

A search did not yield a clear answer to visiting CPT or JNB on a 5 continent RTW. The general trip I am thinking about is: (Stops are in bold)
nrt-hkg-jfk-sju-lax (part 1)
lax-dfw-anc-dfw-lax (part 2)
lax-lhr-dxb-lhr (this is where I would like to go to CPT or JNB)
lhr-syd-drw-per-hkg-sin-nrt

or put another way
nrt-hkg-jfk-sju-lax-dfw-anc-dfw-lax-lhr-dxb-lhr (how to go to CPTor JNB and then) syd-drw-per-hkg-sin-nrt
The validator does not allow LHR-CPT-LHR

Thanks for any suggestions.