Piers,
this might be the wrong place to discuss it, so a moderator might want to move it to a better section.
But anyhow: I am surprized to read about an intentional 'loose' fit of the TSUKA. That would contradict all my technical understanding of this subject.
As far as I know, a blade's NAKAGO has to have a perfect and secure, three-dimensional fit in the TSUKA for safety reasons. It is - at least in my understanding - not the MEKUGU that secures the blade, but the tight fit. The MEKUGI only ensures this, but there should be no physical load on it. With a good TSUKA, you can hold a sword upside-down without MEKUGI! I have always been warned to swing a sword the TSUKA of which is old, shrunk and possibly splitted.
I have also learned that in the SAMURAI era, TSUKA were often replaced, as the life of the warrior depended on the functionality of his sword.
Is that outdated knowledge?