If you think Germany was going to leave or give Belgium territory back or respect it's sovereignty in any meaningful way if they win a European land war with France. Weather or not that involves Britain. I've got a bridge to sell you!
Send a price quote then because gaining access through a neutral territory has a long history in European warfare. Especially in Germany/Central Europe which used be a patch work of states. It was often impossible to fight a war without crossing some neutral territory and rulers often considered granting such passage the wiser option.
Such passage of forces was usually negotiated beforehand and sometimes done with a bit of bullying but I can only recall one example of a nation misusing a passage of forces agreement to take over a country and that was Napoleon in Spain in 1807.
In hindsight, was Belgium really better off rejecting Germany's demand to pass through its country?