The weight of the keyboard is one thing. I have seen students put a stack of textbooks on top of their MacBook laptops without any damage. I can't tell you what the weight was nor do I want to embark on a testing procedure to find out what its weight tolerance is.
However, what I have found is that it's not so much the weight, but the activity that impacts the MacBook. I have seen MacBook laptops used as everything from a cafeteria tray to a seat cushion to even a mirror for putting on makeup. These machines were returned to our depot for repair because of what they was done to them, not what was placed on them.
In your case, you want to use this as a keyboard stand for a mechanical keyboard (gaming perhaps?) so you will be "pounding" on this keyboard for any length of time.
The (rethorical) question is - is this worth it? Is a damaged top case worth this activity? I can tell you that these assemblies (keyboard, case, track pad, etc.) are not covered under Apple warranty when damaged and it's quite expensive to repair. I'd find another solution.