If you're going to brew this in two separate batches, you may consider taking a few pounds of grain from the first batch and adding them to the second batch instead. This way your initial batch is a bit easier on the yeast. then once they're roaring away you can hit them with the higher gravity wort (which would end up getting diluted by the initial 3 gallons).
Oooh, I like that idea. Make the first 3-gallons lower gravity than the target for you barleywine (and by the way, more appropriate for a starter "step"). And make the second higher than target so it all comes out right in the end.
Be interesting to play with the numbers. For something as big as a barleywine, could you get the first batch gravity low enough to not overly stress the yeast, and could you get the second batch gravity high enough to make the end result come out where you want?
Guess that could also make hop utilization calculations pretty hairy but, then again, a barleywine isn't really all that much about the hops.
If your first batch is around 1.070 and your second batch is around 1.140 that works out to 3@70 = 210 + 3 @ 140 = 420 (hurray!) = 630/6 = 6 @ 105. pretty good for a barley wine. and while 1.070 is a bit high for a starter it should be okay in terms of growing up some good yeast. but the same math would work with 3 @ 1.050 = 150 + 3 @ 1.17 = 660.
The other benefit of doing incremental feeding is that the initial 1.050 wort will have dropped to say 1.025 so the actual gravity after the second addition is only 3 * 25 = 75 + 3 * 170 = 585 = 1.097 so you have never exposed your yeast to a gravity higher that that AND the ethanol level stays quite low till the end of the fermentation.