First, a small correction for the OP. The employer can actually offer a plan that covers the spread between the minimum allowed deductible and the max OOP, not just between the HSA maximum contribution limit and the max OOP. This is a common plan and referred to as a post-deductible HRA and is similar to the concept of a post-deductible FSA.
To answer jpod's post: This is not at all inconsistent with HDHP/HSA code/regulations. "Other coverage" is only disqualifying if it pays/reimburses before the minimum allowed HDHP deductible. These are 2017 = $1300 Individual/$2600 Family and 2018 = $1350 Individual/2700 family.
Finally, while totally self-insured plans are usually only the province of very large employers, an HDHP/HSA/(post-deductible and/or limited HRA) can be very cost effective for the employer and beneficial for the employees. Self-insuring a higher deductible above the minimums and the maximum OOP will cost far less than having an insurance company do so. After all, the insurance company is making a profit. Many medium -> large sized businesses can benefit from such constructs.