Dinant luggage storage

Dinant is pretty small but unique; it sounds like you are just going there on a day trip.
If that is the case you can store it any of these train stations that you may be coming from and going back to :

Brussels Nord, Brussels Central & Brussels Luxembourg.
Antwerp Centraal, Bruges, Blankenberge, Charleroi Sud, Gent Dampoort, Hasselt, Knokke, Kortrijk, Leuven, Liège Guillemins, Mechelen, Mons, Namur, Oostende, Ottignies.

Namur is on the same line train line as Dinant

My problem is we are only stopping for the afternoon between Brussels and Luxembourg City.

In cases like this you could subtract one night hotel stay from Brussels and add it to Dinant.
It would be interesting to see Dinant in the evening and in the day as I only went on a daytrip.

Dinant looks amazing. Just got back from Belgium and missed it. Not sure that Rick even mentions it. Wish I had seen it.

Dinant is a charming little town with an interesting citadel. It's well worth an overnight stay. Some friends of mine were in exactly the same situation as you are, and after researching Dinant decided on the overnight stay option. I don't think you would regret it.

We spent a night in Dinant last June at the La Merveilleuse by Infiniti Resorts. It is in a great location overlooking the town and had free parking. We walked into town from it.