i had a similar problem with a custom component with a textinput inside him. My solution was create a public var, something like this:
//This is my component MyComponent...
[Bindable] public var fTabIndex:int = -1;
<mx:TextInput id="field" tabIndex="{fTabIndex}"/>
(Other component...)
//Other component
<mx:TextInput tabindex="1"/>
<control:MyComponent fTabIndex="2"/>
I hope it will be helpful.