A lot of corporate insurance is self funded by the company, with the insurance company being paid for administration of the plan rather than underwriting.
I suppose it is possible that the buyers of these plans agree to link the payments to the cost of the care provided, but I doubt it.
I used to work for a company that built claims benefit management systems, for both direct insurers, and then TPAs (third party administrators).
The flip side of what you say is this - employers are not actuaries in the world of healthcare. So, while an employer can say "hey, whatever else we're doing, we want to give every employee a massage a week, covered 100%, no copay" and the TPA will facilitate the pricing of that, for the general spectrum of care, they will say "We want basically this level of care" and really just choose a plan already provided by the insurer, because all the actuarial effort has been done and the employer has less risk of getting slammed with a multi-million bill because of unexpected incidences.
I suppose it is possible that the buyers of these plans agree to link the payments to the cost of the care provided, but I doubt it.