Think I might have found the ultimate sandbagger/cruiser on Zwift.
The guy who I mentioned previously who was listed as a D despite his top three results all being mid-C class; so after posting about him on the Zwift forums (didn't mention any names, just flagged up that there might be a calculation bug in ZP), the admins took a look and manually promoted him to C. They also retrospectively DQ'd him from my race, meaning I now have the win - wasn't expecting them to do that, but cool that they did.
So I went back onto ZwiftPower today to take a look at his profile and saw that he's raced precisely once since then and finished with the fastest time in a B race, averaging over 4 w/kg for 20 mins, which means in a matter of two days, he's gone from "D almost C", to C and now to "B almost A"! Possibly realising he's been rumbled, he's now decided to race at what was his correct pace all along.
And yet for the whole month of August he's been entering a D race almost every day, winning them all of course, while making it look like he was racing at his limit and so avoiding the two (possibly three) levels of promotion that he was capable of. Just goes to show that if someone REALLY wants to win, there's no way any algorithm can stop them.