Not sure how advanced your PHP coding skills are but if you look in the file administrator/components/com_jevpeople/model/people.php at the _buildPublicContentWhere method you'll see there is some code that allows you to filter the list of people based on the value of a custom field and it already has the filter to search for people with a title containing specific text (more general than starting with)
It would be possible (though not trivial) to create a new dummy custom field type whose sole purpose is to filter the list based on the first letter of the name of the person/resource. Doing it this way would mean you would not have to customise the model file.

Does this sound like the sort of coding you would be able to tackle. If not then your could change the model file and keep a careful note of your changes to maintain them as you upgrade the addon