ajapale,
dont see a problem here. If they get a bonus in say Feb and decide to invest some of this into AVC as a payment that month, the employer should be capable of deducting the AVC under the net pay rules and thus the member gets tax relief immediately. The only problem might be that if by the end of the tax year (Dec) the total AVC exceeded the allowed % of earnings (15%, 20% 25% etc).
The % limits apply to total taxable earnings in the tax year and I see no problem if the actual deductions from salary are lumpy (not spread evenly over the 12 months).