It's easy enough update the order status from the frontend using:
<input type="hidden" name="action" value="commerce/orders/updateStatus">
<input type="hidden" name="orderId" value="{{ order.id }}">
<input type="hidden" name="orderStatusId" value="{{ order.orderStatus.id }}">
But I'm wanting to add a message to go with the status. Any pointers? It looks like the message is part of the Order History model, not the Order Status model.