i understand the frustration of meeting planners not being responsive, and we do all we can to encourage action. We use SMS, phone calls, and email to reach out to them. It's an automated system so nothing falls through the cracks.
We often get last minute requests, for a variety of reasons (speaker replacement, crisis speakers, last-minute events, etc.), so we can't put a restriction on postings. In situations like this, travel expenses are generally not a big concern to the planner. Many speakers are ready to go at a moment's notice. I'd put a tentative hold on your calendar, which means that you're open for another guaranteed event. You can always offer a planner the right of first refusal.