Going on a whim here but would you not be able to use http://developers.marketo.com/rest-api/endpoint-reference/lead-database-endpoint-reference/#!/Activities/getLeadActiviti…
If you set the "activityTypeId (integer): Id of the activity type" on the web page visit for that specific lead record. The response would provide you the asset ids?
Definitely not sure if this helps but potentially worth a try?
Thanks for the answer Floyd,
Unfortunately that endpoint just returns the same information I already have still with nothing to connect it back to the landing page asset.