+1 for the super rigid set up (depending on the shape of the casting and where the slot is, CI can be very brittle so NO vibration at all while machining please); and also + 1 for VERY small/light cuts.
I was surprised to hear Wallace talk about 24 K rpm for router speed, but from his past posts he obviously knows 'is onions. I guess a lot depends on the router cutter spec itself (up or down cut, parallel, number of teeth, etc, etc) but personally - never having tried routing CI - I would have been inclined to go slowest speed possible. But just me being an old woman probably!
But CI is easy to machine (lathe, shaper, mill, etc) AND is also dead easy to file too. Personally I'd be inclined to use a decent file - if the slot is long then pre-drill a couple or three + times then enlarge with a round file - much less chance of accidentally damaging/cracking the casting that way.
If you do machine, as CC has said, you won't get swarf like from machining MS for example, but it can be an abrasive "sticky dust" so do protect the area (old sheet, towel, etc) first.
I for one would be interested to see the job, and hear how it went please.