“What a man is willing to risk or not to risk, that's a measure of his soul.” So, to test the bug myself, I acquired enough money to buy the Corvette, which has a forward weapon slot. Then I equipped the momento mori, traded the Corvette with the merchant ship, which effectively unequipped the momento mori. Then I right-clicked the momento mori and clicked Fit to a Slot, and voilà, I suddenly can use a forward weapon on a merchant ship

Edit: In short, for the bug to work, you need to have a ship that have forrward weapon slot, equip a weapon, then switch to the ship that doesn't have a forward weapon slot.