If you check the surplus shops (I like American Science & Surplus) you can
find cheapy (likely Chinese) diamond wheels at decent prices (as well as
other Dremel bits).
Just remember to polish the cut edges -- important to prevent future
chipping.

With a diamond coated wheel at sufficient speed you'll cut through the
tempering. The trick is to take your time -- don't try to cut too deep too
fast. Better to cut a shallow line all the way along (on both sides, if
possible) and then go back and deepen the line gradually (from both sides),
not letting one section get too much deeper than the rest. This will help
control any tendency to crack as well as Keeping the cutting bit cool.
Polishing will smooth out any irregularities as well as rounding/buffing the
edges to avoid chipping.