Can your mash tun hold enough grain to brew a 10 gallon batch of a normal gravity beer? If so, you can use an iterated mash to hit your target gravity. Mash, remove grains, then use the wort from your first mash as the liquor for a second mash with the rest of the grain. I do this for 3 gallon batches and it works great. However, I'd imagine that the logistics may make this a lot more challenging for a 10 gallon batch.
The best part is that if you pick the right mash temps you can end up with a very attenuative wort. You can perform your initial mash in the alpha amylase temp range, then hold the second mash in range for a low beta rest that overlaps the range for limit dextrinase. The initial alpha rest provides a lot more substrate for limit dextrinase. This leads to a highly fermentable wort, which is exactly what you want for a big barleywine.
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