Short story:
There is no way to set-routing-mark in ROS v7 routing filters.

Long story:
We've got 2 edge routers with eBGP connections to the Internet. Each router is only receiving the default route from the provider. We need to redistribute the default route from each router to the other router using iBGP and place the received default route to a custom route table, so we can use it for policy routing.

In ROS v6 this can be done by setting originate-default to if-installed and by set-routing-mark action in routing filters for the iBGP, but we were unable to achieve the same in ROS v7. There is the rtab property in ROS v7 routing filters, but unfortunately, it is readonly.

I know this doesnt directly answer your question, but it may impact how you approach the ver7 config (maybe looking for routing-table??)
..
In VER6 and VERS7

One creates the routing-mark in some mangle rules OR table in a routing rule........

add chain=prerouting in-interface=bridge new-routing-mark=useWAN2
add src-address=192.168.88.0/24 action=lookup table=useWAN2

IN VER6

NEXT
/ip route
add dst-address=0.0.0.0/0 gwy=ISP2 routing-mark=useWAN2

++++++++++++++++++++++++++++++

In VER7

NEW ADDITION - need to create table.
/routing table add fib name=useWAN2

NEXT
/ip route
add dst-address=0.0.0.0/0 gwy=ISP2 routing-table=useWAN2