function OverrideEntityMeta(ent, meta)
    local REAL = getmetatable(ent)
    local META = table.Copy(REAL)

    table.Merge(META, meta)

    function META:__index(key)
    if META[key] then
        return META[key]
    end

    return REAL.__index(self, key)
    end

    debug.setmetatable(ent, META)

    return ent
end

TEST = create("models/props_borealis/bluebarrel001.mdl")

OverrideEntityMeta(TEST, {GetPos = function(self) return VectorRand() end})

TEST:SetPos(there)

print(TEST:GetPos())
-- Vector(-0.15799432992935,0.2692037820816,0.97308266162872)

print(Entity(TEST:EntIndex()):GetPos())
-- Vector(-3031.0412597656,-2705.6176757813,566.84332275391)

-- :(