Our Personalized Touring Plan software supports the "big" character greetings that have posted wait times, are continuously scheduled, and have well-managed queues (for example, Meet Mickey at Town Square Theater at the Magic Kingdom).
Unfortunately, we do not have the necessary data about schedules and wait times to make reliable predictions about other character greetings.
One workaround is to add a break to represent the chunk of time that you'd like to spend meeting characters. For example, you might decide to spend two hours starting at 11 AM, and you could create a 120-minute break starting at 11:00 AM to represent that. Greeting characters is not generally a time-efficient thing to do, so be sure to be patient and to give yourself plenty of time.