can wholemeal bread flour be used instead of white bread flour for baking soft pretzels?
They will have a less-smooth texture from the bran in the wholemeal flour. You should never swap out more than half the flour in a recipe for wholemeal; the bran will tend to cut the gluten strands, making it hard for the bread to rise, and if you don't have enough gluten to begin with, you'll be in trouble. :)
Whole meal flour is much higher in protein (gluten) than bread flour in order to compensate for the cutting effect of its bran's sharp edges - 14 to 14.5% compared to 12%. The risk is that your pretzels will be tough as a result. For truly soft pretzels with the added benefit of higher fiber, I'd suggest replacing not more than 25% of your bread flour with whole meal flour. And be prepared to add some additional hydration as well. Please let us know how they turn out.