Usage billing for coverage item

Currently only subscription item product type support usage based billing but we have a use case where we want to use Usage based billing for covered item,

Our client is basically a Bus Maintenance Workshop, we have SLA signed with one of our customer who has 129 buses, and we have created a Standard coverage for the Customer, The requirment is we want to add Buses as covered item and also we are charging price on per KM basis, How we can achieve this billing and covered item requirement using coverage item

Currenlty we have mapped this process by creating 129 Subscription item to sales product type as subscription and manually add them in Subscription, but this is very time consuming as we have multiple SLA, how can we do it using Coverage item using covered asset features.