I have a workaround you can try. But you may not like it since it's manually configured iptables rules you add to Custom Rules (/etc/firewall.user). Using ACCEPT in the postrouting_wan_rule chain means it won't reach the MASQUERADE rule last in zone_wan_postrouting.
iptables -t nat -A postrouting_wan_rule -d 172.16.0.0/12 -j ACCEPT
iptables -t nat -A postrouting_wan_rule -d 10.0.0.0/8 -j ACCEPT
Yes in the zone_wan_postrouting chain. But RETURN wouldn't work in postrouting_wan_rule since it would return to zone_wan_postrouting where the next rule is MASQUERADE. (postrouting_wan_rule is the chain where you may put custom rules.) Would using RETURN in zone_wan_postrouting instead of ACCEPT in postrouting_wan_rule make a difference anyway?
I understand what you mean but for me UCI would aways ignore the order of configurations and parameters. I would expect the translation to be like mikma's solution. If it's supposed to work the way you say it works then there is no use for this parameter to accept lists.