I eventually got my external SanDisk Extreme 900 portable SSD drive converted to an APFS High Sierra developer beta 1 startup volume, but it wasn't easy. Here are the details:
I originally installed High Sierra beta 1 on the SanDisk without converting it to APFS, and it successfully booted as the startup volume for my 15" MacBook Pro (Late-2016) with Touch Bar. Very easy.
Later, I restarted and held down Command-R to boot into the SanDisk's restore volume. I then ran High Sierra's Disk Utility and used its Edit menu to convert the SanDisk startup volume to APFS. Disk Utility reported success. Also very easy.
The trouble started when I tried to reboot into the APFS SanDisk startup volume from the restore volume after converting it to APFS. When I chose the SanDisk as the startup volume. I received an alert saying 'You can't change the startup disk to the selected disk," giving the reason "Building boot caches on boot helper partition failed." I then chose to restart into the Sierra internal startup volume but immediately held the Option key down so I could choose to restart into the SanDisk restore volume again instead. I did this several times, and I ran the High Sierra Disk Utility's First Aid each time in an attempt to fix the reported problem. Disk Utility always reported success. Nevertheless, it still would not let me restart by choosing the SanDisk as the startup volume, sometimes giving the reason "Building boot caches on boot helper partition failed" and other times the reason "Running bless to place boot files failed." Booting back into Sierra on the internal drive instead, and then trying to choose the SanDisk startup volume from the System Preferences Startup Disk pane, failed and gave the same error reasons. When Option-restarting as a last resort, I sometimes saw the SanDisk restore volume but not its startup volume, and other times I saw both the SanDisk startup volume and the SanDisk restore volume.
I finally chose to Option-restart and boot into the SanDisk startup volume instead of the SanDisk restore volume, on a lark, and -- it worked! I was at last successfully running High Sierra booted from the SanDisk APFS High Sierra external startup volume.
I guessed that the system routines on my MacBook Pro that run when I try to choose a new startup volume might have had some kind of problem recognizing an external APFS High Sierra drive as a valid startup volume. I noticed that doing a Get Info on the SanDisk in both High Sierra and Sierra Disk Utility reported "Bootable No". Get Info does properly report that it is an APFS volume.
Now that I had the APFS High Sierra SanDisk running as the startup volume, I tried using the High Sierra System Preferences Startup Disk pane to restart into the SanDisk startup volume again. This time it did not report an error and let me proceed, and it booted successfully.
I then used High Sierra System Preferences to reboot into Sierra on the internal drive, and then as a final test I tried using the Sierra System Preferences Startup Disk pane to restart using the APFS High Sierra SanDisk startup volume. It worked, and all is right with the world once again. (Well, almost all. Disk Utility's Get Info still reports that the SanDisk drive is "Bootable NO", but I assume that is an HFS+ thing that I no longer have to worry about.)