For vendor bills I seem to be able to edit the tax total at a header level (so even if it precalculates a number from tax codes applied at the line level, I can override it), this doesn’t appear to be the case for expense reports. The field isn’t available on form customization, so I’m wondering if this is a restriction specific to expense reports or if there’s anything I’m missing?

You are not missing anything..its not available for ER

thanks!