This can be done by utilizing hidden fields and formulas. For your example, what you could do is have a duplicate field for Requestor Email Address that is hidden - this field would have a formula in it that would basically be =IF(a=1, {your variable name for the shown Requestor Email Address}, ""), then on the field with the lookup rule, instead of using Requestor Email Address, you would use the duplicate that gets populated with the formula. That way, it will only have a value if a=1 and the email address is populated, and if a does not equal 1, the lookup will not run.