If there is some timezone which shows up with a name that is not understood by DAViCal, you can add a translation for it into this list:
$c->timezone_translations = array( 'Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London' => 'Europe/London' );
This would mean that any incoming events which had a TZID of "Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London" would have the Olson timezone of "Europe/London" applied for the purposes of calculation. Note that the event itself would not be modified, so your client software will still want to understand this timezone.
Available from DAViCal version: 0.9.9.6 and later