Grimbergen is indeed no Airport of Entry. You can fly to Ostende, clear customs there, then on to Grimbergen but that will probably take more time than the train from Antwerp to Brussels.
Also, Grimbergen has a rather short (about 600 meter, partly grass) strip. You mentioned eight hours endurance. I don't know what plane you're flying but check your landing and take-off distance if you go there. And you need to be comfortable operating a tight and low approach & circuit, as Grimbergen is located in a Special Rules Zone within the Brussels CTR. Get the telephone briefing and put LONDI in a GPS if you have one.