I don't think it's infeasible that air could displace the lifting gas that was compressed, even in a rigid envelope. You could maybe build flexible bladders on

I don't think it's infeasible that air could displace the lifting gas that was compressed, even in a rigid envelope. You could maybe build flexible bladders on the inside for this purpose. Another way to add weight using the same principle would just be to compress outside air into rigid tanks on the craft, making it heavier.