Assuming that this signal (SPI speeds) does need to briefly cross
AGND, should I put a stitching cap there?
Sort answer, yes.
The real answer is: It depends. It depends on how the return current is going to get back to its source and how much current there is. If its a few mA's of switching current, it will cross the plane and return to the source through AGND. If AGND is starred off the board (you'll make a nice loop antenna), or is the ground is not a sufficiently low impedance then you'll get common mode noise of the current going down the resistance of AGND and returning back to the source. If the measurement your trying to make is below uA range, this could be a problem as a few mA's multiplied by a tens of mΩ will give a common mode voltage bump in that range.
The stitching cap is to equalize the return current while its being dumped onto the AGND side and help prevent the short term voltage bump that the AGND plane might see from switching. The stitching cap should be placed next to the digital traces as the return current's value is highest there (due to mutual inductance from the trace adjacent to the plane).
I prefer not to have AGND and DGND separate, if you read Otts book he mentions that in most cases split ground planes should not be used. Use a "bridge" somewhere on the board. (I prefer slots with the "bridge" on one side, but I have good EMC control)
Source HOT consulting Mixed signal Design
If the SPI switching current is low, then you probably wouldn't have to worry about common mode noise.