There is no way to specify bandwith on the PIX at all. On the other hand, BECN and FECN are methods for controlling traffic between your router (DTE) and the ISP that provides the frame circuit (DCE). Those are Frame Relay congestion controls, so that, this are specified into a router (since PIX has
only Ethernet interfaces, not serials). In conclusion, the only way to get QoS in the VPN tunnel is with a routing device, not a PIX.