OpenVPN via MetaRouter OpenWRT

I know that UDP,LZO is unsupported in ROS and probably will never be but like case like that it is what I really need.
I know this is not OpenWRT forum, but maybe someone have such a working configuration or can point me to some proper OpenWRT help.(OpenWRT wiki is a mess)

I have RB2011 set up as pppoe client(I connect to my ISP with pppoe) and I have home local LAN 10.1.1.1/24. What I need to do is direct traffic going to some networks and coming from some LAN clients (10.1.1.99 for example) via OpenVPN on OpenWRT. First part is easy to set up, but the problem I have is OpenWRT.

I know how to setup OpenVPN Client on OpenWRT but I do not know how to setup its networking with RouterOS so I can achieve the scenario described above, ie. “WHEN SRC_IP==10.1.1.99 and DEST==155.1.2.0/20 ROUTE VIA OpenWRT.OpenVPN.Client” or even “WHEN DEST==155.1.2.0/20 ROUTE VIA OpenWRT.OpenVPN.Client”.