Woot. I managed to get it going with an MTU of 1452. Everything's happy now. It wasn't *quite* as simple as just changing a setting in the config file (that was part of it, though), but not as bad as I was expecting (i.e. I didn't have to hunt down the source code to the non-roaring penguin pppd implementation of pppoe that I was using). Thanks for the suggestions everybody!

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