Hello Andy
Yes, it is possible to use the neighbor command with sub-interfaces. Each sub-interface is considered a separate entity when it comes to configuring EIGRP neighbors. Each sub-interface, just like the physical interfaces, can be configured to either automatically achieve neighbor relationship with an EIGRP neighbor, or achieve it manually via explicitly configuring the IP of the neighbor.
Now in the example that you state, the situation is different. Here you have three routers connected to the same subnet. When you configure a static neighbor adjacency between R2 and R3, then R2 and R3 cease to attempt to make any neighbor adjacencies dynamically. Any hello packets sent by R1 are ignored. Whenever the neighbor command is implemented on an interface, that interface cannot dynamically create EIGRP neighbor relationships, and this is why the adjacency between R1 and R2 is broken.
Now this example uses only the physical interfaces, so no sub-interfaces are involved. But even if they were, the EIGRP configuration on each subinterface is independent and would not affect the operation of OTHER subinterfaces, even if they are on the same physical interface.
I hope this has been helpful!
Laz