I don't see the point in even trying to do what you suggesting. Bypassing the PIX when it goes down is just asking for problems, cause if someone found out about your setup, all they'd have to do is crash the PIX somehow and rather than then have no access to your hosts, they'd have full access. Your asking for trouble even trying to do this, and to be honest I can't think of an automatic way to make this happen, you'd have to manually add in a router or something when the PIX dies.
Your management should be made to understand that if they're not willing to purchase a failover PIX (which by the way is a LOT cheaper than a full-blown PIX), then they're going to have no communication if it goes down. They should also be made to understand that bypassing the PIX is a major security issue that shouldn't even be thought about.