What I want is to set the time at boot time from a dvb-c signal (a one-off event) - but I want to do that not at some random time but at a time after syslog is available but before certain other services start start.

By using a sysv init script I can express this order via LSB-entries which makes it pretty simple.

In that case, you don't need a daemon at all. LSB-ordering makes sure that your init script runs at the right time. In the init script, you simply run a program to read the time signal, blocking. No need for a daemon.

Do you think daemontools still have an ecological niche on Debian-based systems now that systemd is taking over?

Systemd is IMHO an ugly monster, stuffing everything and the kitchen sink into one fat binary (or two) on which the entire system depends. Just the opposite of what made Unix great: Little programs that can be combined like LEGO to build things that nobody did think of when they were written. Daemontools follow the Unix spirit of little programs that do one thing, and do that right.

Using daemontools does not mean that you have to remove SysV init, BSD init or Systemd. You could do so, and use a very tiny init that just starts a script to make sure the system is ready to start deamontools, and then passes control to daemontools. But svscanboot can be started like any other service from some init script, or directly from init. This is also the way I use daemontools: I added sv:2345:respawn:/command/svscanboot to /etc/inittab.

Alexander