Here is the response I received from the TAG website.
The limit for term life insurance policies is (less than) 25% of the contribution. This is because term life insurance has no cash value; it is strictly death benefit protection. All other limits applicable to life insurance in qualified plans is derived from this basic 25% limit. I'm not aware that IRS has specifically ruled on universal life insurance. However, since the 25% limit applies to the incidental benefit (the pure life insurance), the amount of a premium that goes towards a plan investment (of any type) is not included in the 25% incidental benefit limit, because a plan investment is not an incidental benefit. This is why the limit applicable to whole life insurance premiums is (less than) 50%. IRS deems half of a whole life insurance policy premium to go towards the cash value of the policy. The other half of the premium is an investment of the plan, and therefore is not included in determining the incidental benefit limit; it is not an incidental benefit.