Q: Is a straight through reverse proxy possible with ELB? For example, a Wordpress instance that lives at my domain.com/blog. Is it possible with ELB similar to apaches mod_proxy passthru functionality?
A: ELB does not support this feature. Suggestions: Create a sub-domain like blogs.crafsy.com and have a separate DNS record + ELB for it.
Q: Is there a way to force traffic to a particular backend app server when fronted with ELB?
A: ELB supports sticky sessions for ensuring that traffic within the same session flows to the same box. For software release purposes, you could have a subdomain like test.craftsy.com and a separate DNS record + ELB.
Q: Can you have a zone apex in Route53 point at multiple ELB's? I assume this is so. (e.g. craftsy.com -> ELB1 (production a), ELB2 (production b)
A: Yes, if you use non-simple routing policy (weighted, latency or failover).