Talked to a good agent who tried lots of variations, but in the end we weren't able to do it with 1 upgrade in each direction.

I ended up using 3 RPUs to upgrade YYZ-IAH-HNL, GUM-HKG-SIN (waitlisted) and HNL-IAH-YYZ; I'll take my chance for CPUs on SIN-HKG-GUM since I had some RPUs to use up.

Couldn't get a 100% definitive answer on the reasons, but it seems to boil down to it being so many flights/regions.

aside: at the time I booked, the W fare was $1800 vs my $1000 K fare.