If I could verify that no additional anchors could be retrofitted, and it was a case like you say (4 children all in harnessed seats, all needing to be ff) I *might* (and mean MIGHT) consider attaching two tether straps to one anchor (my skin crawls even as I type that). Knowing that the anchor would probably do it's job upon the initial impact, and deform towards the end of the crash sequence (ie: I highly doubt it would break entirely), and knowing that the CR using the non-standard angle anchor would probably get pulled terribly to one side (as in uneven forward/side movement)... It would be a very tough situation to be in. I would need to get verification that there isn't some sort of exemption for families like this, and like I said, I'd need to verify that additional anchors could not be added. In my mind's eye I'm picturing (for example) a Chrysler van where the captain's chairs have anchors, and the 3rd row has the centre anchor and that's it. In a case like that I'd do my darndest to get the 2 seats in the 3rd row side by side.. in hopes the seat in the middle *might* assist in keeping the outboard seat from being pulled (ie: tipping over) towards the anchor. Again, this is not a situation I'd want to be in as a tech... and I say again, this is only if all other avenues have been explored. I would probably 'stop the presses' and call Transport Canada prior to making any move at all... having the parents either wait, or return later. I'd consider this a very unique case, and call on any *senior* techs I know. Basically I'd tread very carefully, and use every resource I can think of to make the least worst decision.
Hows that for a very humbled answer.