Nothing is going to tell you the MAC address of something that is not within your local broadcast domain! You'd have to get some code to run in the local broadcast domain of the remote IP address and have it forward you the information.
And PPP links don't have MAC addresses. A MAC address is an Ethernet thing and you don't run PPP over Ethernet.
So I would assume that the original question was asking for a MAC address of an Ethernet (and non-PPP) adapter that was not local to the computer but that was in the local broadcast domain -- nothing else makes much sense. So ARP is the most logical answer.
Actually.. MAC addresses are not an Ethernet thing. Token Ring uses them too... The reason Remote networks are usually harder to get MAC address for could be because of Hardware Address Translation. It all depends on how the routers are set up.