A single user contributing across multiple accounts is already a solved problem in SourceCred. Consider that I contribute both through this account (which we can call discourse/decentralion) and my GitHub account (which we can call github/decentralion).
This is why I wrote the identity plugin: it allows collapsing an arbitrary set of user identities into a single combined identity. So in the example of your forum, you could add an identity mapping that collapses discourse/random_admin and discourse/burrrata into one identity. (I maintain this for SourceCred, how to do it isn’t documented anywhere though.)