Not likely that they'd run the service on the fast lines (which don't have capacity for a service turning off at Slough), so the only solution would be to provide a flyover at Slough to reach the slow lines. The latter is extremely unlikely on any sort of cost benefit basis, there may not even be room to do it and descend in time to call at Slough.
Basically whoever decided to pair the four track railway 'by use' rather than by direction, made through services impossible with train services at today's frequencies.
Paired by direction would still need grade separation for the up branch to join the up relief. The choice of how to widen was no doubt swayed heavily by branches further out being on the north side and having through London service, avoiding the need for expensive flyovers or fully flat junctions which with PBD may be nearly as bad conflict-wise as a flat junction to an opposite side branch in PBU. Think old Hitchin, Woking today, etc.
Slough is a complicated site to upgrade the branch access from the London direction. The major road over-bridges nearby and station footbridges effectively preclude any rail flyover ideas. The station with it's extensive buildings and canopies is heritage listed, making any changes difficult to justify let alone a major reconfiguration, Reading style. If a flyunder solution was proposed it would probably need a new set of low level platforms for the branch with the ramp and junction to reliefs being London side of the station. Very expensive, and what would it achieve? Windsor already has a direct London service from the town's Riverside station. More practical perhaps would be to put the mains into a bored tunnel under the station without any platforms. Then the relief lines could have exclusive use of all four platforms including filter manouvres turning on and off for Windsor. Still eye-wateringly expensive though for the singular benefit of joining Windsor to the Paddington local or Crossrail network, considering the town already has that direct existing link to the capital with it's excellent connectional links at Richmond, Clapham Jn, Vauxhall and Waterloo.
What could be done with the Windsor branch instead? Here's a thought: A train from Windsor reverses in one of a pair of west facing bays at Slough, then proceeds west a short way along a new single track to the south of the Mains. Having passed beneath Stoke Poges Lane bridge, there's nearly a kilometer before the next overbridge at Farnham Road, probably sufficient for a low speed flyover to cross the mains and drop down between the Reliefs. After a junction to both reliefs and a possible new station at the east end of the trading estate, calls could be made at Burnham and Taplow. At Maidenhead the trains would branch off to Bourne End and Marlow. That could tie together the two main towns in the Maidenhead and Windsor Unitary authority and link them both to the employment opportunities of Slough town centre and the trading estate with a more frequent service. As a further stage in the future, tram train vehicles to replace DMUs could allow cost effective lightly built extension of a successfully growing local service to High Wycombe. Just a thought. Crayons safely locked away now!