I was told that Belgocontrol only accept flight plans of flights departing or arriving in the Belgium airspace but I still have to check this.
My experience with the Dutch homebriefing.nl site and the Dutch FIO telephone number is that they do accept flight plans for flights that are wholly outside the Netherlands but within Europe. But you have to be based in the Netherlands somehow before they'll allow you to use their service. (And I don't know what the exact criteria for that is.)
After all, it would be very cumbersome if you do a grand tour of Europe to sign up with all the local ATC providers and their tools just so that they can do the flightplan for you.
The other alternative is of course to use the old-fashioned paper form and have the local tower/info/radio fax it. They'll know where it needs to go for it to be accepted.