Multi-city bike rental in Europe (NL, BE)
I didn't think this was possible, but Google came up with this site (in Dutch) https://www.fietsverhuur.nl

Apparently they deliver rental bikes in Belgium, The Netherlands, Germany and Luxemburg. I assume they can also arrange a pickup at a different location, if the price is right .

It isn't clear if this is just for groups, but you could contact them if you want.