Can a plan include all compensation paid to a participant, even compensation earned prior to the participant's participation date for purposes of determining the 15% deductible limit for 2001?

The 15% deduction limit is based on 415©(3) compensation without regard to what the plan definition is. If the plan does not count pay prior to participation, it does not matter for the deduction limit. If the plan does not count bonuses or OT or something else, it does not matter for purposes of calculating the deduction limit.