Found a really easy solution!

Create a file /etc/pm/sleep.d/70-check-recordings.

In this file check if a recording currently is active. If yes, just return exit 1. That's it. Suspend will be aborted.

My script looks like this, it is based on this at tvheadend.

$ cat /etc/pm/sleep.d/00-check-recordings

#!/bin/bash

#

# this script sets ACPI Wakeup alarm and stops standby if a recording is active

# safe_margin - minutes to start up system before the earliest timer



# bootup system x sec. before timer

safe_margin=60



# modify if different location for tvheadend dvr/log path

cd ~hts/.hts/tvheadend/dvr/log



######################



start_date=0

stop_date=0



current_date=`date +%s`



for i in $( ls ); do

        tmp_start=`cat $i | grep '"start":' | cut -f 2 -d " " | cut -f 1 -d ","`

        tmp_stop=`cat $i | grep '"stop":' | cut -f 2 -d " " | cut -f 1 -d ","`

#       logger "$0: $i from $(date -d @$tmp_start) to $(date -d @$tmp_stop)"



        start_extra=`cat $i | grep '"start_extra":' | cut -f 2 -d " " | cut -f 1 -d ","`

        stop_extra=`cat $i | grep '"stop_extra":' | cut -f 2 -d " " | cut -f 1 -d ","`



        let tmp_start=$tmp_start-$start_extra*60

        let tmp_stop=$tmp_stop+$stop_extra*60

#       logger "$0: $i from $(date -d @$tmp_start) to $(date -d @$tmp_stop)"



        # if recording is active, immediately stop suspend 

        # tmp_stop > now and tmp_start < now 

        if [ $((tmp_stop)) -gt $((current_date)) -a $((tmp_start)) -lt $((current_date)) ]; then

                name=`grep -h -A 1 title $i | grep -v  title | sed 's/.*: "(.*)"$/1/'`

                logger "$0: Currently RECORDING $name. No Suspend until $(date -d @$tmp_stop!)"

                exit 1;

        fi



        # only check future recordings

        # tmp_stop > now and tmp_start > now 

        if [ $((tmp_stop)) -gt $((current_date)) -a $((tmp_start)) -gt $((current_date)) ]; then



                # take lower value (tmp_start or start_date)

                # (start_date = 0) or  (tmp_start < start_date)

                if [ $((start_date)) -eq 0 -o $((tmp_start)) -lt $((start_date)) ]; then

                        start_date=$tmp_start

                        stop_date=$tmp_stop

                        name=`grep -h -A 1 title $i | grep -v  title | sed 's/.*: "(.*)"$/1/'`

                fi

        fi

done



wake_date=$((start_date-safe_margin))



# set up wakeup alarm

if [ $((start_date)) -ne 0 ]; then

        logger "$0: Set Wakealarm for $name to $(date -d @$wake_date)"

        echo 0 > /sys/class/rtc/rtc0/wakealarm

        echo $wake_date > /sys/class/rtc/rtc0/wakealarm

fi

Now set up KDE to handle standby after time-out. If a recording is active, KDE standby is aborted by the above script.

See also this in the MythTV documentation.