Should have checked this earlier I guess, but I tried about 30 synths being lvl 88 Clothcraft. 0 skill up. 88-89 in 27 synths, 3 Wamoura Silk into Wamoura Cloth after that.
Wamoura Silk (synthed from cocoons, not the cloth) caps at 88. This has already been confirmed by every crafter I've seen who have attempted to skill past 88 on it.
I second that. While I have not tried crafting it past 88 i got a skill up on a failure when i was 83 and it is my understanding when you are 5 lvls below cap you get skill up from failure.