You said it yourself, Dwarves are mildly immune to magic, and are unable to cast spells. Thus the idea of labling a Dwarf a mage is about as pointless as naming the Dog one.
It's established canon that due to living under ground all these years has overexposed Dwarves to raw lyrium, causing some immunity and the inability to touch the fade, thus removing any chance of that "Dwarven Mage". Sandal might come close, but then Sandal is half elf (or so I've heard), and even then he's more like a Tranquil then an out-and-out magic user. As well, yes they can work with Lyrium for runes and enchantments, but for most it's very dangerous work that takes years and years of study to perfect.
Now a Runemaster could very well be a thing a Dwarf could be, at some point in the future of the dragon age series -- Dwarves who've carved lyrium into their very skin, akin to Fenris, that can be activated at will to give combat strength, but even that has been said to be lethal, not to mention the only person to ever have it done was an elf (who are naturally more attuned with magic) and the person who managed to pull it off is dead.
All in all it doesn't really fit with the story we've been given, and honestly I kind of adore the idea that Dwarves just aren't meant to do magic.