I am interested in something similar to this and was thinking that integrating pfBlockerNG would facilitate creating an access list to be used for routing purposes. In this case I would think that adding the domain to pfb would resolve all of the ip's for that site/domain and adding them to an access list, then setting a routing statement using that access list as the destination to route through the vpn instead of the WAN.
What I am wanting to test is using pfblocker to create an access list for the .onion domain, then routing the traffic destined to that domain through a vpn. For instance, there are ubuntu repos on tor, and when updating packages from that repo, I would like that traffic to automatically route through the vpn connection instead of attempting through my wan.