Warlock 4/ Sorcerer 1/ Eldritch Theurge, and qualify with Versatile Spellcaster from Races of the Dragon along with any Bloodline feat (page 112+) other than a native bloodline. A few of those won't cost you any spell options, but you could pick any of them and use your invocations to cover what it takes away, such as taking Serpent Bloodline if you plan on using Fell Flight. Another idea would be to see if taking Fey Bloodline could be used as a substitute for Fey Heritage for example, so you could take Fey Power and maybe Fey Skin without spending the extra feat. That could also work for Fiendish Bloodline/Heritage, or just go with whichever one is best suited to your character's origins.