According to my quick research, this should work...
<?php
function wm_item_pre_post_session_edit() {
$key = 10; // Your meta field ID.
$meta = Params::getParam('meta'); // Get all meta fields.
if(@$meta[$key] == null) return; // Stop the function if your specific meta field doesn't exist. @ is added to stop warnings if $meta is not an array.
$old_value = $meta[$key]; // Get value from your meta field.
$new_value = 'New value'; // Put new value for your meta field.
Session::newInstance()->_setForm('meta_'.$key, $new_value); // Save new value.
Session::newInstance()->_keepForm('meta_'.$key);
}
osc_add_hook('pre_item_post', 'wm_item_pre_post_session_edit');
Regards.