Generally the water-based polyurethanes are the lightest finish you can apply, typically described as "water white", if not blue-ish. Other things to try may be much less durable unless you can get into a catalyzed lacquer that is typically only available in large commercial settings because of the complexity of the equipment and ventilation required.

You might try a wash coat of dewaxed super blonde shellac (~1 lb cut) topped with a water-based polyurethane. It will add a touch of amber. Nitrocellulose lacquer can be very clear, but will give some amber tone. You also need a proper spray setup to use it safely.

It might be worthwhile to test a couple different water-based polys; they are similar but not identical in their properties. General Finishes products are an alternative to the ubiquitous Minwax. You might also look at the Varathane Crystal Varnish, or if you have spray equipment a CAB-Acrylic lacquer.

Keep in mind that no matter what you do maple will develop its own amber tone over time as a natural property of the wood.

Thank you very much for your input rwiegand!

I am wondering whether that natural ambering over time with maple would be expected even following ferrous sulphate treatment? What about following bleach treatment?

I will investigate your other finishing suggestions. Lacquering is not out of the question as I would consider hiring a pro to do that if I can be confident of the outcome.

I believe most of the yellowing that occurs in all woods over time is due to oxidation of lignins, one of the primary structural components of the wood. This process can be slowed by finishes that block UV light and free access of the wood to the atmosphere but I know of no way to stop the process.

I found that Rubio Monocoat in 5% white was the magic bullet. It provides a topcoat that doesn't change the colour of the underlying wood (stained or not) at all.

Here is a link that might be useful: Rubio Monocoat Natural Oil Finish 2C Part A