I do not understand the "ERISA v tax code" question. It is all about tax.
I think you should look at the award from the perspective of section 72(m)(10) of the tax code. The language is difficult to unravel and the terminology sucks, but I think the intent is to allocate basis proportionately to the allocation of balance. One might guess that a purpose of 72(m)(10) is to prevent allocation of basis disproportionately to the higher-rate taxpayer. From that I conclude that one cannot allocate only basis to an alternate payee or allocate a designated amount of basis (meaning an amount determined in some way other than pro-rata).
I think Q&A-9(b) applies only after the division and identification of the alternate payee's interest, which means after the application of section 72(m)(10) to the division.