That’s why it’s a TrueNAS feature request, which happened to be accepted and put on their roadmap.
If TrueNAS, through its UI, can integrate (as a first-class / tested feature) importing a read-only pool, it allows for easier recovery operations for those who are unable to import their pools. No command-line needed. “Pool doesn’t import? Try it again with the read-only box checked and see if you can recover and copy your data somewhere else.”
It also works in tandem with the checkpoint feature, which I also made a request for. You could rewind your pool non-destructively if you check the read-only box during the import wizard.