andersonpg

Thanks! That pointed me in the right direction. If anyone else is interested, here is the code I used via a snippets plugin to limit the time zones:

<?php
add_filter( 'tribe_events_timezone_choice', 'limit_timezone' );

function limit_timezone( $timezone_selector_html ) {

    $retain = [
        'Honolulu',     // UTC-10
        'Anchorage',    // UTC-9
        'Los Angeles',  // UTC-8
        'Phoenix',      // UTC-7
        'Denver',       // UTC-7
        'Chicago',      // UTC-6
        'New York',     // UTC-5
    ];

    $html = '';
    $lines = explode( "\n", $timezone_selector_html );

    $i = 0;
    while ( $i < count( $lines ) ) {
        $line = $lines[$i];

        if ( 0 === strpos( $line, '<optgroup ' ) ) {
            // Look ahead to check if the optgroup is empty
            $j = $i + 1;
            $optgroup_empty = true;
            while ( $j < count( $lines ) && 0!== strpos( $lines[$j], '</optgroup>' ) ) {
                if ( 0 === strpos( $lines[$j], '<option ' ) ) {
                    foreach ( $retain as $acceptable_timezone ) {
                        if ( false!== strpos( $lines[$j], ">$acceptable_timezone<" ) ) {
                            $optgroup_empty = false;
                            break 2; 
                        }
                    }
                }
                $j++;
            }

            if (! $optgroup_empty ) {
                $html.= "$line\n"; 
            }

            $i = $j; 
        } elseif ( 0 === strpos( $line, '<option ' ) ) { 
            // Check if this option is in the $retain list
            foreach ( $retain as $acceptable_timezone ) {
                if ( false!== strpos( $line, ">$acceptable_timezone<" ) ) {
                    $html.= "$line\n";
                    break;
                }
            }
            $i++;
        } else {
            $html.= "$line\n";
            $i++;
        }
    }

    return $html;
}
?>