It's been done and the results were not good.
I'm going from memory here so specific details might be incorrect.
Around the early or mid 1990's there was a guy that took a passenger on a tandem. I do not remember the details of the 'problem' but the passenger died from the landing. The TM suffered injuries too, but recovered. This happen at a small central CA DZ (Arvin?- IIRC).
The family of the deceased sued EVERYONE!
eg: the TM, the tandem rig owner, the pilot, the DZ, USPA, the tandem Mfg (Strong) etc
The TM was NOT rated by Strong - even though he used a Strong tandem rig. I believe he did have a RWS tandem rating (or did have one at one time). This was when the FAA wavier was still in effect. There was no USPA tandem rating. USPA also had TMMI (aka student insurance) at the time.
Months & months go by with all parties spending TONS of money to 'get out' of the case.
USPA's insurance company 'paid off' even at the request of the organization NOT to do this. This made it even harder for Strong to show that they were not responsible for this accident. [An innocent paying off looks like admitting guilt.]
Strong went to court hearing after hearing trying to convince the court that this guy was NOT Strong rated nor the owner of the rig and therefore Strong was NOT liable.
In the end, after lots of money was spent (~several hundred thousands of dollars) Strong was found something like '10%' liable. They had to pay more money.
IIRC, the reason for this '10%' liability was something to the effect:
'You have the wavier that says you monitor your TM/owners in the field. You did not do this adequately, otherwise the tandem rig owner would have never let an un-rated person use the rig.'
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