@Renee Snow "I simply can't think of a way to transfer this W2 income to X other than a negative "other income" adjustment, nominee distribution for X's SS#."

Well then, just do it.  But transfer the withholding credits also, as the PLR instructs (although they contemplate the ISO stock being transferred to X before the sale, so this only mentions the NSO's:

"11.   X is entitled to the credit for income tax withheld from the stock or cash proceeds (or paid to Company by X for properly due income tax withholding) at the time of the exercise of the nonstatutory stock options of which X is beneficial owner."

This is essentially what you would do if they were still married and filing separate returns that split the community income and withholding.   What would happen if you filed a Form 8958 with a return marked "Community Income Split" ?  At least it would route processing to the group that knows what you're dealing with. 

The Form 8958 instructions tell us:

This form is intended for individuals who:
1. Are subject to community property laws, and
2. File separate federal income tax returns.

If the shoe fits, wear it.