Since no one answered, I will detail the solution we came up with:
On the sub-site calendars we created a new column for each list called Share With Master, then each calendar has a simple workflow when an item is created with the following logic:
If Share With Master = Yes
{
Email Content Manager [of the Master Calendar] a link
}
The link sent to the Content Manager contains each of the field values filled out in the querystring. (it might look something like /newform.aspx?Title=test%20title&Location=test%20location)
The link opens newForm.aspx of the Master Calendar and we use jQuery to populate each field of newForm.aspx so the Content Manager just clicks save and "approves" the information. To parse out the above example we would do the following:
//get parameters
var title = getURLParameter('Title');
var location = getURLParameter('Location');
//remove characters like %20
var parsedTitle = decodeURI(title);
var parsedLocation = decodeURI(location);
//assign to fields
$("span input[title='Title']").val(parsedTitle);
$("span input[title='Location']").val(parsedLocation);
This doesn't give us the bidirectional functionality we were looking for but does allow us to easily share events between calendars. Hope this might lead someone to a better solution.