config.tmpl

<mtapp:setting
    id="dynamic_setting"
    label="Dynamic Setting">
    <input type="text" name="dynamic_setting" value="<mt:var name="dynamic_setting">" />
</mtapp:setting>

config.yaml

name: Plugin Name

settings:
    dynamic_setting:
        default: 0

init: sub { my $p = shift; require PluginName::Plugin; bless $p, 'PluginName::Plugin', $p;  }

config_template: config.tmpl

Plugin.pm

package PluginName::Plugin;

use strict;
use warnings;

use base qw( MT::Plugin );

sub load_config {
    my $plugin = shift;

    $plugin->SUPER::load_config(@_);
    my ( $param, $scope ) = @_;

    if ( $scope =~ /^blog:(\d+)$/ ) {
        my $blog_id = $1;

        # tweak $param as needed for values
        # to pass to the config template

        # for example, set dynamic_setting to the current time
        $param->{dynamic_setting} = time;
    }
}