How to get access to tap0 (ovpn-client) interface on OpenWrt from host?
for example:
Mikrotik 192.168.1.1
OpenWrt 192.168.1.100
tap0 192.168.2.10

first you create virtual ethernet or assign ethernet interface to metarouter guest. Then you can create openvpn tunnel (or any tunnel) over the network connection, and then you can either use already existing ethernet or create new one and pass all packets passing TAP0 to ethernet.

I can ping remote network 192.168.2.0/24 and local 192.168.1.0/24 from OpenWrt.

Afrer:
/ip route add gateway=192.168.1.100 add dst-address=192.168.2.0/24
i can ping 192.168.2.100 (tap0) from host, but cannot ping the gateway (192.168.2.1) or any other device on the 192.168.2.0/24 network.

throw away all the "virtual" stuff and what you get in the end is - 2 networks that have to be routed. Set up routing correctly and you should find that everything "just works"

Just treat all the "virtual stuff as an actual stuff that is connected by wires.