That is what I've learned already and I understand it halfways.
What I do not really understand is: why is there no option to trigger a script by SMS as least? It is true that I could implement a timed polling mechanism via cron and gsmctl and then send commands directly to /dev/rs485. But I do not like the long minimum polling interval caused by the use of cron and needing to reinvent the wheel by creating my own Modbus master. But anyway, there is a first possible solution.
If there was a possibility to trigger scripts by SMS, one could modify the serial Modbus master configuration in order to send out different commands to the Modbus slaves. It could be accomplished by UCI over SMS already but this is not to be used by the end user.
The reason why I was mentioning virtual I/O: every existing tool inside the TRB can handle I/O. If it was possible to create such virtual I/O and use it as a variable source inside the modbus data, then that would be a most versatile solution without focussing too much on SMS.
I haven't had a look at the SDK but I imagine that I could not extend the existing webui with it. I mean, I could not extend the existing V2.04 with custom dialogues. Or if yes, would there be a chance to get such modifications into the standard product? It would be bad if a customer wiped out all by updating the firmware.