For early-game, two thematically appropriate ones (in my mind) are Florist and Healomancer. Others have extolled the virtues of a Florist - the themes match up with poison, plants, natural allies in woodland areas, etc. - but I'd say a Healomancer works just as well. Protecting and caring for her Dwarves, making them loyal to her, and just as useful early-game to keep an army alive when she's on the front lines with them. It fits with the Snow White ideal in my mind, something relatively gentle and sweet, but that could be used to support something that's most certainly not. The problem is of course that late-game both of those fall off a bit. Both excel when fighting on the front lines, buffing allies and disabling enemies, but aren't much scratch if you lock 'em in a tower away from the fighting.
For sheer utility, there's a lot that suit someone who's sitting in the fortress late game and not doing much. Mathamancy, Moneymancy, Predictamancy, Lookamancy, Thinkamancy, Turnamancy... but some of those are more optimal than others if she'll be leading armies early-game, and some are more thematically appropriate. A Luckamancer or Mathamancer, for example, could be quite useful to fudge dice rolls and cheat the system in a way that is the fairest of them all. The Dwarves are of course miners, so Moneymancy makes perfect sense. The Mirror saw things nobody else could, so Lookamancy or Predictamancy fits.
Personally, if I want the ideal Caster Overlord to sit in a castle and not die, it's a coin-toss between Moneymancer or Dirtamancer. But if you want a thematic match, a good early-game Warlord, and a good late-game Caster to sit in your castle... Turnamancer is my vote. Snow White turned the Hunter, earned the friendship of all she met, and was the fairest and most beloved so that all adored her. It fits the theme, it's handy on the front lines, and it's handy to sit in your base and pump out new units.
But of course that's just my opinion.