It will be nice to have some wrapper for sysfs and procfs (/sys, /proc). It will remove the necessity to use the postconfig script to set thoose values and have the setup benefit from config management.

For instance, i need it to control IGMP behavior of a bridge:
"IGMP snooping support is not yet included in bridge-utils or iproute2, but it can be easily controlled through sysfs interface. For brN, the settings can be found under /sys/devices/virtual/net/brN/bridge. "

For this particular use case, there is two way of managing this:
1- A global wrapper, run when the configuration is fully loaded.
example :

set system sysfs 'devices.virtual.net.br1.bridge.multicast_snooping' 0

that writes 0 to /sys/devices/virtual/...

2- Something attached to the interface.
example :

set interface l2tpeth l2tpeth1 system 'brport.multicast_router' 0

That write 0 to /sys/devices/virtual/net/l2tpeth1/brport/multicast_router
and is applied each time a commit is done on the interface.