We actually recommend exactly this to many of our customers. Unless the marketing homepage needs a lot of dynamic app-database driven content we usually recommend installing wordpress on www.domain.com and rails app on domain.com.
when user is not logged-in, domain.com redirects to www.domain.com.
this setup saves a lot of time on static pages management and adds quite a bit of marketing power.
Whale Communications, before they were purchased by Microsoft, had a hardware solution where a shared HDD disk was used to physically switch it between 2 different hosts to enable file sharing in a shared directory. would be almost perfect to secure the hot wallet of a BT service. MS seem to discontinued the product. I wander if something like this still exists from another vendor?