In short, Clique will do exactly what you want it to =).  Clique provides an extra button on your spellbook, when this is open you are presented with a list of your current click-casts, and a few options ("New" and "Cancel") being the only two available.  From that point, you simply click on spells on your spellbook, using the modifiers you'd like the spell to be bound to.  For example, if you Control-LeftClick on "Rejuvenation" (That's where I have it bound) then it'll appear in the spell list as "Control-LeftButton" and the click-cast will be bound for any frames you have enabled.

Once you have a spell bound, you can click the "Max" button, and it'll remove the "RANK" portion of the spell, always casting the highest rank (or in the case of a lower level target, drop only BUFF spells down to the lowest level, just like in WatchDog.  Beyond all of this, you can click the "New" button and it'll pop up a blank spell with an lua edit box.  The edit box allows you to enter arbitrary LUA (not /cast commands, but anything that comes after /script or appears in a mod).  The edit box has syntax highlighting and coloring which makes it nice and easy to edit the code.  This means to make a Nature's Swiftness/Healing Touch bound to a key:

1. Open the Spellbook
2. Click the Clique tab
3. Click "New"
4. On the "binding" button, set the click cast to whatever you'd like, but simply clicking on it.
5. In the name field, type "NS/HT"
6. In the lua edit field, type:
CastSpellByName("Nature's Swiftness")
SpellStopCasting()
CastSpellByName("Healing Touch")
7. Click "Ok".

Now on any frame that Clique is providing click-casting for, any spells and custom code you have defined will work =).  Right now I am NOT providing any heal selection logic, but I'm not entirely against someone providing me with a library to handle the rank selection-- I just have no intentions of writing a spellbook parser and writing all the logic to handle selection.  In short, Clique will clearly define what happens when you click on any of the unit frames you have up-- I aim to support every single unitframe mod that a user wants me to-- the plugin system is pretty robust.