No new information on the Discord regarding the soundtrack and artbook, unfortunately.

But I wanted to share something I just realised. Even though it really only confirms what we already know, maybe this info could be useful to somebody in the future (feel free to ignore this post otherwise):

When you preorder a game that includes preorder DLC on GOG, your "orders history" will show you buying the game on the day you paid for it. But on the day the game gets released, your preorder DLC will show up in your orders history tab as a free purchase. It will show the DLC as a separate product, with the price attached, but discounted 100%.
So, for example, the Mercenary class preorder DLC for Biomutant shows up in my orders history as a free purchase on release day. It's named "BIOMUTANT the Mercenary DLC" and its price is listed as "$3.58", crossed out and marked down to "$0.00". This suggests that the Mercenary DLC has already been set up within the GOG database, with a proper name and a price of $3.58 associated with it. Presumably, once the devs decide to start offering that DLC for sale to non-preorder customers, GOG staff can simply uncheck a box somewhere in the backend and the DLC will appear on the store, for the price it's already listed as.

Now, the contents of the Necromunda: Hired Gun preorder DLC on GOG are identical to the Hunter's Bounty Pack. But the entry for the preorder DLC in my GOG "orders history" is simply called "Necromunda: Hired Gun - Pre-order DLC" and its price is "$99.99", marked down to "$0.00". If you go to SteamDB, you'll find that Steam games which have exclusive preorder DLC that won't be sold after release will have this DLC listed as a separate "preorder DLC" product (compare Iron Harvest and Darksiders 3, for example). According to SteamDB, Necromunda: Hired Gun does not have any products in the Steam database called "preorder DLC". This indicates that people who preordered on Steam didn't get a separate "preorder DLC" product, they simply had the "Hunter's Bounty Pack" product automatically added to their account for free on release day.

Which basically means: Someone at Streumon/Focus Home/GOG made the decision that they wouldn't offer the Hunter's Bounty Pack on GOG - and they made that decision before release, not afterwards. They created a "preorder DLC" product in the database with a dummy price of $99.99. This was presumably done only in order to be able to offer the same preorder bonus on GOG, Steam and EGS. And since they never intended to sell the Hunter's Bounty Pack on GOG, they didn't need to give it a proper name and price in the database.