On further research, it appears that there is more flexibility under the law than I had thought. However, as a practical matter, it can be hard to play pick and choose. The insurer wants a lot of healthy gamblers both for revenue and to spread the risk out. So, although the insurer is unlikely to say "no," they may well say "no" under any partciular plan where it is not cost effective for them to offer the plan to small numbers. Or, in other words, it is legal but expensive to give special coverage to a small subset of employees of a small company.
The large, publicly traded company plans with which I am familliar do not have separate eligibility rules for people at least up to the VP level. What they do is offer everything to everybody and provide a benefit allowance separate from straight wages or salary. Anything above the allowance is a deduction from otherwise expected earnings. Naturally, the benefit allowance goes up with responsibility and/or tenure. So, if a customer contact center telephone service representative wants to get the executive package, he or she can get it if he or she earns enough above his or her benefit allowance to pay for it. For individuals who have other insurance at least as good as what their benefit allowance pays, they receive the allowance as pay. Although I have not worked at 50% of fortune 500 companies, I understand this is a popular approach both because it is provides equal choice to all employees and, because of the concerns in the paragraph above, it's probably cheapter to offer it that way.
Thanks for the correct info, HRforME.