the extensions are in the booted /tmp/tce/optional directory
It would be a better idea to unsquash it along with its dependencies directly to the root of your remastered corepure64.gz
And then create empty files in the /usr/local/tce.installed/ directory to prevent reloading.
Assume that you are talking about scsi-KERNEL.tcz (scsi-4.8.17-tinycore64.tcz)
It does not depend on any other extensions.
So you won't need other extensions copied.
Also, make sure to add an empty file scsi-KERNEL (scsi-4.8.17-tinycore64) to /usr/local/tce.installed/
but they do not load on boot
You'll have to dig into the start-up scripts to get the answers.
My guess is that, by default /tmp/tce/optional is empty, thus the tceonbootload is skipped.