Looking for the "private vlan proxy arp" feature too, see RFC 3069, supported since Linux 2.6.34, can be enabled by /proc/sys/net/ipv4/conf/*/proxy_arp_pvlan . If ROS 5.x is based on 2.6.35, it should be fairly easy for Mikrotik to add this feature.
I have a few wireless APs each with client isolation enabled, connected to a switch with "protected ports" feature which basically does the same between different APs, yet would like to allow some devices in the same IP subnet to communicate through the router despite the L2 isolation. All these APs and stations connected to them have to be in the same subnet, as the stations can move between AP without IP address changes (if one sector fails, customers can be moved to adjacent sectors with weaker signal but better than nothing). Can't replace the switch (Netgear GS110TP) with a Mikrotik bridge as it has other unique features (802.3af PoE output, external 48V DC power supply, reasonable cost).