Routing traffic to different gateway on remote site

Hi!

I have two sites- Site A and Site B. Need to somehow mark client-b (from site A) traffic so that it is sent to isp-c (on site B)
Both sites are connected over L2TP and running RouterOS. But gw-vpn is running OpenWRT.

Problem is that “client-b” is defined by Address List on gw-a, so I cannot use hardcoded addresses on gw-b. I was thinking about using DSCP for marking these clients’ traffic so that gw-b can route traffic based on DSCP mark, but it is necessary to keep existing DSCP marks untouched.
Another solution I was thinking was to use some sort of Tunnel or VPN between gw-a and gw-vpn, but I really hope there is better way to do it.

I’m not really sure which is the best way to achieve this scenario. Please see diagram attached.
sites.png

You can try add additional address to L2TP on site A (and of course corresponding on site B) and NAT client-b to this address. On site B simple route this address to isp-c.