@halftrainedharry finally looking at this again.
The solution to automatically create a slug (alias) from the Event title turned out to be straight forward.
In my existing custom migx update.php Processor I noticed a comment //overwrites on line 312 with examples of how to overwrite items in $postvalues.
So I added the following to generate the required slug.
if (($classname == 'VenueEvent') && (empty($postvalues['slug']))) {
$postvalues['slug'] = $modx->filterPathSegment($postvalues['title']);
}
A better solution might have been to mirror the same functionality MODX uses to set the alias field using the pagetitle for a Resource but I couldn’t figure how to do this inside a MIGX update window.
The relevant code seems to be translitAlias: function(string) { in \manager\assets\modext\widgets\resource\modx.panel.resource.js, however even with your helpful video about MIGXdb Customisations I didn’t figure it out.