AFAIK samurai would never sit in seiza with their long swords in their belts, it would instead be placed on the side or disallowed while inside certain places and only able to carry their wakizashi (short sword).
You think so? Have you seen many samurai movies? Have you seen sword drawing skills done from the knees?
When it comes to etiquette and the sword even the placement was based on the fact of not being able to take it as if drawing it. Once again...a reason for.
If you have reality on drawing a sword from the knees then you will see it's no different from standing.
That's why it's safer and indeed vital for a samurai to sit so in readiness.
That fella in lone wolf and cub demonstrated it quite well.