On page 212 of UCamp, under the construction heading, "At the GM's discretion, construction magic (such as lyre of building, fabricate, or wall of stone) can reduce a single building's BP cost by 2 (minimum 0)." So, the (optional rule) to use it for kingdom building is already taken care of. For Downtime, maybe a simple rule, " A lyre of building can be used to provide half the labor costs used in construction. If so used, it reduces the time to construct a room/building by half as well." My reasoning being that the lyre can be used as unskilled labor, but some skilled labor must be used as well. But because it produces a disproportionate amount of unskilled labor, that can be used to speed up construction.