If it moved anything approaching 3/4 of an inch it would tear the nails out. There is still some skill to installing strip flooring. Let it acclimate for multiple days out of its bundles and spread out where it will be installed. Tighter in high humidity (it is going to shrink), looser in low humidity (it is going to expand). Strip floors that are to tight for the moisture content at install can buckle up and pull fasteners part way out. The humps then decrease when the humidity falls. Most of the hump will disappear. Trying to face nail (or even screw) the area will not usually work. The forces generated are larger than the fasteners and/or wood can take. Splitting is not uncommon. You have to remember that if you have 5 inch wide strips 10 feet wide you have 24 strips of wood. If each one move 1/32 inch from damp to dry, the total movement you have distributed is 3/4 inch. It only sows as the joints between the strips opening and closing by 1/32 inch though (and yes it is probbaly off by 1/32 since 24 strips only have 23 joints).

If your hallway is usual size for such a thing, there is no reason to provide a 3/4" expansion space. Give it 1/8" at each doorway or 3/16", if you're nervous about it, and then fill the gap with a non-hardening (i.e. flexible) caulk. Solid hardwood that is mechanically attached with nails, staples or cleats will not allow for that much expansion. Worst case would be edge crush, severe cupping and tenting in situations where there is excessive moisture in the environment. And, if there was that much moisture from a severe water event (burst pipe, storm damage...those kinds of things) the floor would probably be ripped out anyhow.

Thanks. I'm also installing in the "humid" part of the year (RH in the 30-50% range), with material that came from an even more humid area ... it has probably done all the expanding it's going to do. And yes, it does seem illogical for that gap to be there. My older rooms (1890s install) have no gaps. At least where the walls are straight there are no gaps. : (

Use a caulk backer rod - 7/8" maybe, and compress it into the gap, leaving 1/4 - 3/8" depth to fill. A textured silicone caulk that matches the brick mortar would probably look the best in this situation. I would avoid filling with any grout or mortar - it will crack with contraction and expansion.