You will need an A number in order to apply for a Re-Entry Permit.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
If you are approved for permanent residence but have not received your green card yet, you can make an InfoPass and request your passport to be stamped as temporary evidence of your status. You A# is indicated on the I-485 filing receipt.