Anyone, and that includes regular employees, contractual employees, ex-employees or even non-employees, can have the freedom to choose the postpaid plan that they want... it's a free country.
What is being asked for is not a Proof of Employment but rather a Proof of Income or just documents that can prove that you would be able to sustain the plan that you are applying for.
Credit Card Statements, ITR to prove that you may have other sidelines or your salary as contractual is enough, Other Billing statements from utilities, Bank Statements, Rental Lease Contracts and other Income Generating contracts, Regular Remitance statements, etc.
And if you do not have any proof of income then you could pay your Monthly Service Fee 6 months in advance or get an existing Globe subscriber with good credit as your guarantor.
Everyone has the freedom to apply and choose a postpaid plan just present all the requirements and let globe study your financial capabilities. GLOBE even offer other ways to get a plan even if you fail to provide all the requirements, like paying 6 months advance of your bill.