Hi Magnus,
Yes, you can create your own statement GUIDs. They are, in essence, nothing but the entity ID and a random number.
$guid = $entityId->getSerialization() . '$' . trim( com_create_guid(), '{}' );
https://github.com/wmde/WikibaseDataModelServices/blob/master/src/Statement/... https://github.com/wmde/WikibaseDataModelServices/blob/master/src/Statement/...
I hope that helps. Sorry, no guarantee, Editing via action=wbeditentity should always work, but you will probably run into the same issue.
Best Thiemo