This may have been discussed before -- I cannot find the answer however. Anybody have thoughts?

I'm going from SOF to TUS in June. I have a choice of issuers (UA, CO) and many, many different routings on this, all about the same price. Sticking with UA metal, virtually all routings are "W" from Sofia thru the US gateway city, then "K" or "L" for 1 or 2 local connectdions to TUS. I'm hoping to use an SWU on this.

Question is: If I book through the US gateway with "W" on UA metal, can I use the SWU to the last "W" leg, or does the entire route have to qualify in order to use the SWU?

I know there is a question of whether a CO marketed, UA metal ticket will qualify -- if that's a real risk, I can use UA issued tix -- the difference is not that great...

Thanks for your help FT'rs -- I'm sure I couldn't get the answer anywhere else!