Hi,
below, I provide a script that addresses my initial use-case. It is a very quick workaround for this feature. I hope that the feature will make it into the next release as I am convinced that this is a very common use-case if you are using duplicati in a mobile laptop environment.
To use the script (linux only), set up duplicati to run very frequently. I tested it with “every 6 minutes”. 30 minutes or hourly might work as well. Point the options “run-script-after” and “run-script-before-required” to the script.
When called before the backup, the script checks if a prior backup run for today was successful. If not, it checks if the remote host is up. if it is up, it lets duplicati run the backup. If it is down, it will postpone the backup to the next scheduled run to retry then.
However, there seems to be a scheduling issue. In case the backup is running and another backup is started due to the scheduler (i.e. the backup runs longer than 6 minutes), the scheduler seems to queue the run. That means that all missed runs are executed in a queue after the backup finished. Does someone know how to stop this?
The check whether the backup was successful still needs some improvement, though 
Improvements are welcome.
duplicati-pre-post-run-2.sh.zip (929 Bytes)
(.zip because it is the only archive type allowed by the forum)