The best way to do what the OP is wanting is to have each spell only have a <t> function. Basically macro'ing all your spells so that if they don't have a target they simply fail. That is how I have everything set up on my White Mage. So my Cure macro looks like this:

/micon Cure
/ac Cure <t>

To the more broad topic of out of range targets that has been brought up in the thread, I am probably in the minority but I feel that indicators whether they are visible only to the healer or not, or people being grayed out isn't needed. When you target someone they have a marker above them so you know where they are. You can set your spells to not default back to you using macros (this should probably be a global option in settings instead of having to macro) and part of the challenge of using spells like Cure III, Medica, and Medica II is positioning yourself to hit as many people as you can. Once the defaulting back to you issue is handled I think the way it works is fine.