I'm curious now too - I've got a Sigma EF-500 DG Super - and happily popping away on RF-602's... The test shot I posted was actually backlight/filled using an Achiever TZ250 (hey - I picked it up for $10 !
) and the key light (if you can call the hot spot a 'key light' lol ) off of the Sigma in manual at about 1/8th from memory..... It wont work in P-TTL mode obviously, or in 'slave' mode, but appears to work fine in standard Manual mode (modelling light even works) - But I'm really just getting into the strobist thing so maybe I'm missing something entirely here... It may also be that mine has had the FW update to work correctly with the K10D/K20D built-in flash wireless P-TTL Slaving which perhaps has 'fixed' the issue as well that you refer to Class A?
Re the cable - glad to hear that your cable is a shutter release - My apologies on the clarification about the cable - Yongnuo don't make a "pentax" kit - just the Canikon ones... But as the canon trigger cable is also useable on the pentax I noted that some sellers sell the canon ones (RF-602/C1?) as a "Pentax" kit in the description.. (but its still just the C1(?? - mebbe C3) kit) The 'standard' kits without a shutter cable supply a similar looking cable, but with 3.5mm jack and its actually the PC-Sync cord (will shutter release, but wont AF on half-press unless you have "AF on remote shutter release" selected in the menu..)...