Cutting a hole in the centre of a beam (spar) should not cause a problem structurally as long as the hole is the smallest size possible, has smooth sides i.e. no crack inducing sharp corners, and is only in the web of the spar and does not touch the flange surface top and bottom, think of a castellated beam, I would be tempted to laminate a second piece of similar size timber to the area with the grain in the opposite direction of the original spar. full width of the gap between ribs on either side of the hole, belt and braces I know, but its the structural engineer in me, happens all the time even though I am now retried.
Mike