I went ahead and added pagination and maxResults options to the
listForProduct and listForProductAndSku methods themselves, and now each
will recurse (if maxResults unspecified) until all results are returned
(no pages left). This seemed truer to the actual API methods, which
simply accept pageToken (page identifier) and maxResults as optional
params.
(https://developers.google.com/google-apps/licensing/v1/reference/licenseAssignments/listForProduct)