If an active duty servicemember is serving outside the United States can their spouse sign their joint tax return on behalf of both of them, if so$,$ are there any special reporting requirements?

Yes, one spouse can sign for the other spouse and no explanation is needed.

No$,$ one spouse can never sign for the other spouse.

Yes, one spouse can sign, as long as there is an explanation why they are signing for the absent spouse.

Yes, one spouse can sign, provided the other spouse has a signed Power of Attorney.