Is the Raspberry Pi 3 Model A+ currently supported?

Certainly,

The image will not boot on the Pi 3 A+. There is no activity at all according to the green LED. If I then move the same SD card to a Pi 3 B it works fine. Additionally, the Pi 3 A+ works perfectly on plain raspbian.

At this point, I suspect it is related to the kernel version. If I do the following on the Pi 3 B:

sudo apt-get update
sudo apt-get upgrade -y
sudo reboot

The SD card will then boot on the Pi 3 A+. Of course, Allstar will now be broken since I did not preface the above commands above with:

sudo apt-mark hold raspberrypi-kernel-headers raspberrypi-kernel

If I upgrade using the Pi 3 B and hold the kernel version by including the line above, we are back to the start, the SD card will not boot on the Pi 3 A+

73,
Karl