Hi Andrew,
As you said, VRFs are deployed to separate naturally networks so they can't exchange information. It also allows the support of overlapping IP addressing plan.
If you want different VRFs to start exchanging traffic, you can on each PE import the routes of the other VRFs and upodate all your FWs rules.
But it's like merging all the VPNs into one and you loose the main advantage of having several VPNs. Also it's not possible to do that if you have addresses overlapping between your VPNs
That's why we prefer deploying dedicated CEs as inter-vpn gateway so you allow the communication but you keep the control. Usually the CE is associated to a FW. This design is commonly used to provide Extranet VPN services.
This design supports overlapping as well if you configure VRF aware NAT.
HTH
Laurent.