No, a charity cannot pay volunteers under the Small Benefit Exemption Scheme.
The Small Benefit Exemption Scheme applies only to employees on the payroll. Volunteers, who by definition do not have a contract of employment or receive regular remuneration, are not considered employees and therefore are not eligible for benefits under this scheme.
Charities can reimburse volunteers for actual expenses incurred in the performance of their voluntary duties (such as travel or subsistence), and these reimbursements can be paid tax-free, but this is separate from the Small Benefit Scheme. Attempting to pay volunteers through the scheme could result in the payment being treated as taxable income if the individual does not meet the criteria for employment.