Animated number counter in Keynote

I answered my own question. Pretty easy actually:

1. View > Edit Slide Layouts
2. Create a new layout (or edit an existing one), adding a "Slide Number" placeholder on the slide layout (you'll have to select this option under the "Appearance" panel on the right side of the Slide Layout view)
3. Style your placeholder number however you want it to appear
4. Exit out of Slide Layout view
5. Add slides, starting from the first slide in the presentation, using the layout you just created: each slide will now show a number. corresponding to its slide # in the presentation.
6. To animate, select all slides, set animation to "Start transition automatically" and the delay according to however fast you want it to play...
7. So... if you wanted a counter that goes from 1 to 100, you'd create 100 slides, apply the same master layout (with placeholder element for slide #) to all of them, and then set them to automatically transition to the next slide at whatever speed you like.
8. PRO TIP: to create a large number of slides, create 10 at first, then select all of these, and paste them at the end of the current slide list. Repeat to 100, then select all, copy and paste at the end of the current list, etc.