worked up some basic stats, with progression so you could see what i'm doing. comments and suggestions appreciated of course.

BTW, wrote this up before Knight Otu's post, so the final HP will be less.
Female advanced half couatl/half gold dragon
a normal couatl:
Large Outsider (Good, Lawful)
Hit Dice: 9d8+18 (58 hp)
Initiative: +7 (+3 Dex, +4 Improved Initiative)
Speed: 20 ft, fly 60 ft (good)
AC: 15 (-1 size, +3 Dex, +3 natural)
Attacks: Bite +12 melee
Damage: Bite 1d3+6 and poison
Face/Reach: 5 ft by 5 ft (coiled)/ 5ft
Special Attacks: Spells, psionics, poison, constrict 2d8+6
Special Qualities: Telepathy, ethereal jaunt
Saves: Fort +8, Ref +9, Will +10
Abilities: Str 18, Dex 16, Con 14, Int 17, Wis 19, Cha 17
Skills: Concentration +14, Knowledge (any three) +8, Listen +16, Search +15, Sense Motive +16, Spellcraft +15, Spot +16, Tumble +15
Feats: Dodge, Improved Initiative, Still Spell
Climate/Terrain: Warm forest
Organization: Solitary, pair, or flight (3-6)
Challenge Rating: 10
Treasure: Standard
Alignment: Always lawful good
Advancement: 10-13 HD (Large); 14-27 HD (Huge)
then, advanced to 27 HD:
Huge Outsider (Good, Lawful)
Hit Dice: 27d8+108 (229 hp)
Initiative: +6 (+2 Dex, +4 Improved Initiative)
Speed: 20 ft, fly 60 ft (good)
AC: 16 (-2 size, +1 Dex, +6 natural)
Attacks: Bite +33 melee
Damage: Bite 1d4+10 and poison
Face/Reach: 10 ft by 10 ft (coiled)/ 10ft
Special Attacks: Spells, psionics, poison, constrict 4d6+10
Special Qualities: Telepathy, ethereal jaunt
Saves: Fort +19, Ref +17, Will +19
Abilities: Str 26, Dex 14, Con 18, Int 17, Wis 19, Cha 17
Skills: Concentration +15, Knowledge (any three) +8, Listen +16, Search +15, Sense Motive +16, Spellcraft +15, Spot +16, Tumble +14 (plus 198 more skill points)
Feats: Dodge, Improved Initiative, Still Spell (plus 4 more feats)
Climate/Terrain: Warm forest
Organization: Solitary
Challenge Rating: X? (does it matter?)
Treasure: Standard
Alignment: Always lawful good
Advancement: ---
finally, with half-dragon template applied:
Huge Outsider (Good, Lawful)
Hit Dice: 27d12+135 (310 hp)
Initiative: +6 (+2 Dex, +4 Improved Initiative)
Speed: 20 ft, fly 60 ft (good)
AC: 20 (-2 size, +1 Dex, +10 natural)
Attacks: Bite +37 melee (give claw attack?), 2 claws + 32 melee
Damage: Bite 2d6+14 and poison, claw 2d8+7
Face/Reach: 10 ft by 10 ft (coiled)/ 10ft
Special Attacks: Spells, psionics, poison, constrict 4d6+14, breath weapon
Special Qualities: Telepathy, ethereal jaunt, immunities, darkvision 60 ft, low-light vision
Saves: Fort +20, Ref +17, Will +19
Abilities: Str 34, Dex 14, Con 20, Int 19, Wis 19, Cha 19
Skills: Concentration +16, Knowledge (any three) +9, Listen +16, Search +16, Sense Motive +16, Spellcraft +16, Spot +16, Tumble +14 (plus 225 more skill points)
Feats: Dodge, Improved Initiative, Still Spell (plus 4 more feats)
Climate/Terrain: Warm forest
Organization: Solitary
Challenge Rating: X?
Treasure: Standard
Alignment: Always lawful good
Advancement: ---