During Salah, can I place Najis leather valuables, such as wallets or shoes, in a small bag and place the bag in front of me so that it is beneath my stomach during prostration?
Yes it is permissible to do that, even you can keep Najis leather or any other Najis items in your pocket during Salah as far as Najis items are not wet because wet Najis item will make your pocket Najis which will invalidate your Salah.
Wassalam.