Any substitution for bread flour?

The reason you need a higher protein content for brioche is because of the high fat (butter, egg yolks) content of brioche. Proteins are long, sinuous molecules. Manipulating them via kneading is what causes bread to literally be held up by them. Fat causes protein strands (gluten is the protein in flour) to separate, to shorten. Brioche is something of a hybrid. It is a bread which has a tender, pull-apart quality. The two occur because of using bread flour which has a protein content of 12 - 12.5% compared to all-purpose flour's 10 - 10.5%, a difference of nearly 20%, which is quite significant. Another reason to use bread flour (which you essentially created by adding vital wheat gluten to your AP flour) is that brioche is full of ingredients which are very heavy: eggs, butter, sugar. It takes a stronger protein to lift them all up.

I'm not sure what there is to be horrified about. I have been baking bread for more than 50 years. I have made brioche many times. And I have never, ever used any flour that was labeled "bread flour," nor have I used "gluten." And I have always had successful results, not only with brioche, but with other breads as well.

So I would say to the OP, use what you've got and don't worry about it.

Since you're probably using a pan for your brioche, and it usually has a fine crumb, you might not notice the difference in a lower gluten flour. Where I have noticed a difference is in free-form (no pan for support) loaves: more gluten seemed to help my baguettes and boules hold a nice high and round shape. Anyone else notice something like that?

I'm sure lots of bakers on this site will be horrified, but I have used all purpose flour instead of bread flour many times. I did a lot of baking growing up when I had no access to different flours, and everything pretty much turned out fine. The texture might be altered slightly but I bet no one will notice unless you do a side by side taste test! That being said, I think the advise above is great, since bread flour does have slightly more gluten than all purpose. But basically, I'm sure it will be great, and don't worry!

Many thanks for your expertise!

You are so in luck with that gluten. To your remaining 1.5 cups of AP flour, add 1.5 tablespoons of vital wheat gluten and be on your way.