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I think it's been done, but I'm not sure of the distances involved. From memory it was the I²C that was used - MartinR certainly used the I²C bus to communicate between stacked emonTx V2s in his "Full fat" 3-phase monitor, so if the bus will drive the distance you require, that should be at least one possible solution.

Sure, about any transport method you choose is possible. Assuming you're using the emonTX V3 code, the data transmission is handled by send_rf_data(). Using a different transport method is just a matter of using different code in that function, and having the supporting libraries for the new interface of course. Whether there's existing software for OEM that'll use the interface you want is another question. If you're willing to put in a bit of work though, it's probably not a large effort.

If Ethernet is of interest, that should be doable, and will take you a further distance than i2c without having to potentially deal with various signaling issues. If you decide to use an arduino ethernet board/sheild and the RFM is still populated but unused, be aware that a complication exists depending on which hardware revision you have for the ethernet board/shield. The SS pin on the Wiznet chip isn't wired correctly on certain arduino ethernet boards/shields. That bug is resolved in current hardware, and there are workarounds published for older hardware, but if you're not using the RFM, may as well not populate it, if possible.