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just an idea when using a boring bar........could you make it extra long so it fits inside the bore of the headstock mandrel. it could have an oilite bush on the end and this would stop any deflection ?? !!! I have not done this myself ...just a bit of inspiration...thinking inside the box/mandrel !!!
The main downside I can see is that you will have to hand feed with the tailstock rather than use the leadscrew or feed shaft to move the carrage under power. If you want to support both ends of the bar then why not just use a between ctrs bar with the work on the cross slide?