Get MAC address from a dead phone
iPhone original packaging should have the Wi-Fi MAC address in addition to the serial number and IMEI printed on it.  An Apple service desk or service technician might be able to look that up for you, but older devices might be out of the recent database or release of that detail might be considered out of scope. 

iOS itself as well as the iTunes back up syncs never need the MAC address, so you would be better looking at Internet logs and router logs and basestation logs since they would record that type of information as a routine auditing function. 

Lastly, diagnostic logs from the device might have that information so if you opted into sharing diagnostics with either a third-party developer or Apple, they might have records of your mac address in their databases of crash reports and support incidents. 

Opening the device would only make sense if you could get it to power up and join the network-I would be surprised if the address or visible to inspection internally