I think the reason for Tony not making an appearance is simple, the servants were never to be the main focus after Season 1 and the fade out of chauffeur Michael Culhane. Once Season 2 started and barrelled along full-throttle, the focus was solely on the trappings of the rich family Carrington. Look how much effort it took to keep getting middle-class Claudia living at the mansion in Season 2!
Tony served a plot point in Season 2 between Krystle and Alexis. Really what else was there for him to do but be the background colour in scenes showing the main cast? Look at Kirby. Once she was introduced it was all about getting the Downstairs Girl upstairs. I agree it might of added layers to Kirby if say she had an attraction to the studly gardener soon to be stable boy. But Kirby's story was destined to be one of a love triangle between the two Dynasty Princes of the time, virtuous Jeff and scoundrel Adam. When we get to Season 8 and Matthew has taken Gerard, Jeanette, and Hilda Gunnerson hostage along with the Carringtons, the scene between Gerard and Jeanette, though well written and great for their characters, feels so out of place because the show for seven seasons prior gave the characters absolutely NO character development outside of anything superficial that served the plot of the main characters.
Most likely during Season 3 there was just no need for Tony, even in the background, for what the show wanted to accomplish that Season. Tony was a welcome sight in Season 4, but he really did not do much. His shirtless scene with Fallon was much appreciated tho!!
And an aside, not to get this tread off the rails, I found this interview with Jack Coleman from 1991 where he discusses his time on Dynasty and his feelings towards the show and his playing Steven. And it gives his explanation as to why he did not appear in the reunion........
TV or not TV. . . . CAREER MOVE: Jack Coleman seems quite happy that "Dynasty" is well in his past.
www.latimes.com