TL;DR -- AC isn't really relevant for ghosts, and ghostly dragons may have serious trouble doing the dragon thing anyway
The normal trick doesn't hold in this case
Dragon-type creatures lack armor proficiency, so the normal means of using ghost touch modified armor to boost a ghost's AC doesn't apply.
What good is armoring a ghost, anyway?
However, ghosts don't need their AC. The rules for incorporeal creatures already incorporate a 50% damage reduction for corporeal spell effects other than force effects as well as magic weapons without the ghost touch modifier, and render the creature totally immune to mundane weaponry. On average, this is equivalent to half the party's otherwise-hits missing outright, or a significant boost in AC.
Are you sure this is a good idea?
Furthermore, many of this ghostly dragon's attacks won't work properly due to the way the ghost template's special attack rules work:
Special Attacks: A ghost retains all the special attacks of the base creature, but any relying on physical contact do not function.
Specifically, it will lose its melee attacks, and its Crush ability. Its breath weapon may still work (do you have to be in physical contact with a breath weapon for it to hurt you?), and its aura and frightful presence still will function, but this can cut the DPR output of the ghost dragon quite significantly.
Worse yet, a ghostly dragon won't have its normal stockpile of HP to play with -- the application of the ghost template to a dragon is a 33% cut to its hit die, and also leaves it leaning on its notably inferior Charisma modifier for its HP modifier.
While you do get extra save-or-X options from the template, the DPR from Corrupting Touch struggles to make up for the potential lack of a breath weapon, and the other options are not terribly useful against a high-level party:
- Corrupting Gaze -- 1d4 Cha damage helps against Cha-dependent classes, but not much else.
- Draining Touch -- while useful to target statistical weaknesses in a party, if you have a party that has strong stats across-the-board, it might not be of much help in bringing anyone down, or even in shutting down a caster
- Frightful Moan -- this otherwise-nice ability is mostly superseded by the longer-range, always-on, much longer-lasting Frightful Presence the dragon already has
- Malevolence -- probably the most useful ability this ghost dragon will have, although it means giving up a fair bit to get there
- Telekinesis -- you already have at-will mage hand and the ability to cast attack spells, so all you get from this is the ability to move heavy objects and grapple people less effectively than you did before you were a ghost