Is there any real reason why the slider events can't be posted to the queue in real-time? i.e. in the same way that mouse move events are?
Mainly because on Windows, as the user is changing a slider's value, the OS enters a modal loop, which means that normal program execution isn't performed - however, event hooks are called from within the modal look and therefore can be used to intercept these events. Only when the mouse is released is normal execution resumed (i.e. your main program flow continues) and Blitz just posts the final slider value in the queue.