I don't think there is any easy way to do what you want to do.
As far as I can tell (I'm a bit new to mikrotik) there are two ways to bake in a default config:
1) provide a default config script at netinstall time. (which really isn't likely to be useful for an ISP to do on a per-customer basis)
2) use the TR069 client and an ACS to put a default configuration into the device.
#2 is the only way I've seen browsing the Mikrotik documentation that might be able to do that dynamically.
The down side to this is you would need to setup an ACS and install the TR069 client on all of your customer devices.
Now, I haven't tried any of this but it is something I myself want to be able to do and it's on my todo list to experiment with.
It seems to be possible according to the documentation, but it's of course not a simple thing to just decide to roll out overnight either.
https://mum.mikrotik.com/presentations/ ... 109593.pdf