Well I'm still playing Unleashed and I got round to trying an industrialcraft "miner"...

First observation: the diamond drill takes far too much power, but I've seen this behaviour elsewhere so its normal for the Unleashed pack and with any luck shouldn't show up in monster. IC2 Experimental has its own quirks like the OV scanner sometimes needing 1000,000 eu to charge (version dependant).

Second observation: Backfilling does not work. This might be a permissions issue since as far as I know this is the only time industrialcraft actually "builds" anything. Instead the miner consumes blocks as though it was backfilling but leaves an empty hole behind. Oddly enough you don't see the "Drill head" block when its retracting either.

What should happen is that as it retracts the drill pipe it should replace the pipe with blocks from the middle slot, building a column. You can even put ladders or elevator rails into the miner and provided the south side of the hole is a continuous wall it will build a ladder for you.

Backfilling did work in Tekkit in the Nether. I don't recall trying it in Tekkit Overworld

On the bright side the Railcraft tunnel bore appears functional now.