How to lower the melting point of a specific chocolate?
I don't think you can lower the melting point without melting the chocolate first.
Butter is very soft at room temperature and adds it's own typical butter flavor. A better alternative is hard coconut fat.
I always use this trick when I cover a cake with a simple chocolate glaze. If you simply melt the chocolate and slather it on top of the cake, it becomes so hard that it's hard to cut cleanly and (depending on the thickness) hard to bite. Of course that has to do with the ruined tempering.
Adding coconut fat to the molten chocolate makes it softer without turning it into a ganache or mouse. For a cake glaze I usually add half the weight of chocolate in coconut fat. The result is very shiny, still has a bite and unchanged chocolate flavor, but melts within seconds in your hand and wouldn't be able to hold its shape without the support of the cake. You'll probably want to add significantly less fat.
To the question of tempering: If you manage to melt the chocolate while baking, the end result won't be tempered anyways. Indeed, untempering can lower the melting point of chocolate without adding any oil or fat. Here's a very detailed blog post about tempering chocolate that explains the different crystal types cocoa butter can form and what their properties are. Tempered chocolate has only one type of crystal that is very stable. If you don't temper chocolate at all you'll end up with a mix of different crystals that might make the texture more gritty, but also lower the melting point.
Unfortunately I cannot offer any more detailled instructions, you'll have to experiment to find the best solution for you.