technically speaking the realtor cannot make any decisions about the property other than when to show it. he is just an agent and unless the agent has permission to give permission, which he probably doesn't, ask the owner , or.....hit and run!....but cover your holes. the realtor probably won't give you the name and number of the owner for such endeavours. instead, look up the address at your counties local tax office website, then reverse look-up the address to obtain the phone number. that easy. if they are listed with a land line you will find them that way.
GL and HH!