SSD + apfs Install option greyed out

It should work by exiting any High Sierra installer GUI and starting the install from the command line:

/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO

Further install options can be obtained by using the --usage argument:

Arguments
--applicationpath, a path to copy of the OS installer application to start the install with.
--license, prints the user license agreement only.
--agreetolicense, agree to license the license you printed with --license.
--rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
--pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
--converttoapfs, specify either YES or NO on if you wish to convert to APFS.
--installpackage, the path of a package to install after the OS installation is complete; this option can be specified multiple times.
--usage, prints this message.
--volume, path to the target volume.

Sources report that the command (in the Beta version) doesn't do anything on the first execution but on the second.

According to this and this article the post-install conversion to APFS should work in Recovery Mode:

1. Boot the Mac into Recovery Mode by holding down the Command and R keys until you see the macOS Utilities screen.
2. Open Disk utility.
3. Choose your boot volume [the external boot drive].¹
4. Click on the Edit menu
5. Click on Convert to APFS
6. When prompted with “Would you like to convert to APFS?”
7. click on the Convert button. When the process is finished, click on the Done button.

¹: According to Redarm's comment you have to unmount the volume additionally!

I can't test this myself - your mileage may vary.