1- You can make a toner by adding tint to some poly and spraying it on. This works to a point as the poly can't hold much tint but it can change the color of finished wood.
2- Apply a coat of gel stain then a coat or 2 of poly. This has the best chance of darkening the oak but is somewhat difficult.
I would try a toner first on a single piece to see if it will get the color your customer wants. To get the toner have a quart of the same poly used slowly tinted and your favorite paint store. Make sure to have a piece of finishes wood to apply samples for experimenting at the store.