Would I be correct in assuming that since Section 137 benefits may be included in a cafeteria plan that such expenses may be reimbursed through an FSA?

It would be a separate FSA from unreimbursed medical or dependent care, therefore a separate election. One word of caution, if adoption assistance is put into the Section 125 Plan, it is subject to the "Use it or Lose it" rules regardless of the circumstances. It might be more prudent to have the employee take the expenses on their taxes if there is any danger that the adoption would not be finalized during the Plan Year.