Also the web of trust should occur among assemblies, and not among individuals. You may read an old thread about this idea.
Objection in the model of the web of trust of Ucoin - Archives / Discussion - Duniter Forum
The alternative idea: Let assemblies elect trusted representatives that will be send to other distant assemblies (actually they will secretly participate in their meetings and observe their procedures).
These elected/trusted (or randomly selected?) representatives will vouch that the procedures of the distant assemblies are followed correctly, and thus a web of trust among various local assemblies (and a web of trust among various local currencies) is created, and a UBI can be given.
Let these representatives be paid in their local currency for their journey. Let the assemblies vote the numbers about the compensation of the representatives.