You might want to also look into iproute2 as a way to dynamically bring up and tear down routes for ClearOS. The multiwan engine uses this and you can simply setup rules to process in the /etc/clearos/firewall.d/local file to bring them up anytime.

I had a situation where I needed to be able to have ClearOS switch from one gateway mode to another depending on a keepalive script and configuration. I didn't have the luxury of using BGP or OSPF so quagga was off the table. The setup fails over the entire infrastructure to standby VPN tunnels (which are always up) whenever the call center admins determine that the primary ISP is not working out well. Originally it was thought that the system would only be used infrequently but the system has saved the company from expensive downtime over, and over, and over again.

If you do end up using quagga, I'd really be interested in talking to you and producing better documentation for that under ClearOS.