A few cautions:
If the plan is a governmental plan, check the plan’s documents and State law.
If the plan is a church plan, check the plan’s documents. Even if no external law calls for a particular provision, a sponsor might decide that providing for a participant’s spouse (or, at least, not depriving a spouse of an interest without the spouse’s consent) follows the church’s faith.
If a plan is neither a governmental plan nor a church plan and the charitable organization believes it is not a plan within the meaning of ERISA’s title I, consider that a disappointed surviving spouse might assert that the plan is an ERISA-governed plan. A Federal court decides a defendant’s motion to dismiss a complaint for failure to state a claim considering only the complaint’s alleged facts, with no opportunity for a defendant to introduce facts. Because of this, a defendant might not persuade a judge that a complaint is so implausible that it asserts no claim.
Even if a plan is unquestionably not ERISA-governed, read the annuity contract or custodial-account agreement. In my experience, some older annuity contracts provide a survivor annuity, absent the spouse’s consent, and provide this even if no plan the contract was purchased under imposes such a provision.