you can mount your .conf on top of the regular one. you just need to make sure everything gets copied first from it to the one you wish to mount, then add your options at the end.... there is two options for initializing scripts, either using the nvram option for on mount usb or using the nvram jffs2_exec option which launches at early boot.
I know this is old thread, but this is for knowledge base.
To be specific, this is what I did, it works great!
I just combine it with entware, resulted in the following
Markdown (GitHub flavored):
```shell
mkdir -p /jffs/scripts
```
Adding script to execute when mounting the usb
Markdown (GitHub flavored):
```shell
cat > /jffs/scripts/mountOpt
#!/bin/sh
# 1. mount the opt dir after every reboot
# 2. change dnsmasq ipv6 dns server
mkdir /tmp/opt
# where /dev/sdX# is your usb opt partition
mount /dev/sda1 /opt
# find and replace line
sed -i "s/dhcp-option=lan,option6:23,.*/dhcp-option=lan,option6:23,[fe80::3]/" /etc/dnsmasq.conf
# restart dnsmasq
killall dnsmasq && dnsmasq --log-async
```
press ctrl+d to exit
Markdown (GitHub flavored):
```shell
chmod 755 /jffs/scripts/mountOpt
nvram set script_usbmount="/jffs/scripts/mountOpt"
nvram commit
```
Restart the router, to verify the result use `sudo radvdump` in linux terminal to see RA message, you should get something like this
Markdown (GitHub flavored):
```shell
sudo radvdump
#
# radvd configuration generated by radvdump 2.18
# based on Router Advertisement from fe80::f22f:74ff:fec4:3415
# received by interface br0
#
interface br0
{
AdvSendAdvert on;
# Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump
AdvManagedFlag off;
AdvOtherConfigFlag on;
AdvReachableTime 0;
AdvRetransTimer 0;
AdvCurHopLimit 64;
AdvDefaultLifetime 600;
AdvHomeAgentFlag off;
AdvDefaultPreference medium;
AdvLinkMTU 1492;
AdvSourceLLAddress on;
prefix 2403:6200:xxxx:xxxx::/64
{
AdvValidLifetime 600;
AdvPreferredLifetime 600;
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr off;
}; # End of prefix definition
RDNSS fe80::3
{
AdvRDNSSLifetime 600;
}; # End of RDNSS definition
}; # End of interface definition
```