I do not wish to repeat what other orgs have said, but I would like to add my contribution on the issue of leather belts (and straps) specifically.
First, we are not banning outright the use of leather, whether for belts or other purposes. Rather we are discouraging the use of modern belts or the traditional 'LARP belt' for costume or weapon support and encouraging the use of a more Rokugan culturally appropriate alternative.
Perhaps it may help to consider the use of leather for costume in our L5R LARP in this manner:
Most period or fantasy LARPs (where appropriate) discourage the use of t-shirts or similar with modern prints and logos, jeans and similarly anachronistic clothing. Players are not banned from wearing them, but are encouraged to cover them with culture and setting appropriate costume. I would hope to address the use of leather belts and straps in the same way. By all means use them where necessary, but where possible cover them with cord or cloth to hide the leather.
From my research online regarding the samurai armour, I find the belt worn to support weapons was likely to be made of cloth or cord, as discussed by other posters.
This 'belt' is often referred to as uwa-obi or himo (himo is the word for tie or cord generally). This uwa-obi is used solely for the purpose of securing weapons, scabbard (saya) and similar items. It is worn over the obi, armour or anything else the samurai is wearing.
The uwa-obi has been made of both cloth or cord, depending on design and era. Often the uwa-obi is long, wrapping two or three times around the body, and is tied at the front.
For the cloth version, the uwa-obi is likely 1- 3 metres long, and about 20 – 25 cm wide. The cloth is then folded over double lengthways (or triple, as you please) to make a strip about 8 - 12 cm wide (give or take from these measurements as preferred). Any cloth can be used for this, it is usually fairly light – silk, cotton, linen, even cheap polypop would do.
I won’t comment on cord, because I haven’t used it or seen it used for this purpose. I agree that cord is likely to dig in and hence I will be choosing the cloth uwa-obi for my costume.
I have used a cloth obi like the uwa-obi I describe above for my naginata training outfit. Kendo and other Japanese martial arts disciplines may use something similar, I don’t know. I found this very secure and comfortable.
When researching online I noticed that in all the photographs of actual samurai I could find – admittedly not too many – the uwa-obi appeared to be of cloth as described above, and all white or light in colour. Of course these were monochrome photos of late period armour (19th century), and don’t represent the breadth of Japanese armour history, nor are they informed by Chinese, Korean or other styles as the fictional Rokugani styles might be.
So for our Rokugani LARP, I hope to be costuming our NPCs with uwa-obi or himo as above instead of belts, and attempting to replace leather in other areas (such as quiver straps) with cloth or cord alternative.