[j-nsp] Installing backup default-route from upstream by BGP condition?

[Markus]

Hi,

on a M7i with 8.0R2.8 I'm receiving a full BGP feed from my upstream,
and a single default-route through a second BGP session with the same
upstream (but to another of their routers) for backup purposes if the
first session should go down. Is there any way that allows to install
the default-route only when the full BGP feed session goes down? 

What I want to achieve is that traffic to destinations which don't exist
in the global routing table won't get sent out to the upstream at all.

Can anyone point me to the right direction? 

Thanks!
Markus