Is it true that Moredhel chests will keep any items put in indefinitely? If so, are there any other similar safe containers in BaK?

You can safely store a single item of any size under the bridge to La Mut and in the wall at the entrance to the sewers underneath Krondor.
Moredhel chests are safe for the most part, certainly safer than regular locked chests or dirt piles.

Don't store things in Locklear's room in Krondor between chapters as they will disappear.

Thanks for the response. Do you know which Moredhel chests are not safe?

In the patched 1.02 version all wordlock chests are 100% safe, as well as all caches inside of city screens. However, a few wordlock chests get new contents in certain chapters.

All other caches are not safe, and are extremely likely to get emptied whenever you leave the current map region.
The least safe caches are bags left near enemies.

As far as I know, James' room IS 100% safe, but not accessible in all chapters, like the wordlock chests in non-reachable map regions.

You can also store things permanently by selling them to shops up to the maximum capacity of the shop (not counting the shops permanent inventories); selling another item to a shop after reaching the maximum capacity will result in another non-permanent item being "deleted" from the shop. Of course you will have to BUY those items back, but you can do so multiple times. Shops will buy only certain types of things, and items already available there.

For the details about the content changing chests, and which locations are accessible in which chapters, and Item types bought and sold by shops, consult the "BAK Help Web".