Pay private bolt11 via sendpay
There is the routes field in the invoice as a routing hint
   &quot;routes&quot;: [
      [
         {
            &quot;pubkey&quot;: &quot;03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134&quot;,
            &quot;short_channel_id&quot;: &quot;1891311x61x0&quot;,
            &quot;fee_base_msat&quot;: 1000,
            &quot;fee_proportional_millionths&quot;: 100,
            &quot;cltv_expiry_delta&quot;: 144
         }
      ]

This tells you the node ID which getroute can find:
03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134
There are plugins that allow you to create custom routes along a given path. So I suggest to call getroute to the node from the routing hint extend that route with the private hope and use the following plugin to create a custom onion:
https://github.com/lightningd/plugins/pull/85