I know you have a reason to lock "List of Counters" field from editing just after PerfCounter Custom creation. It was device internal database structure as I can remember.
Can you please investigate to implement similar mechanism as you have already for SNMP Disk Free sensors: a lock with a description "Warning! Only edit this field when asked to do so by the Paessler support team. Unauthorized use can lead to false monitoring data!" ?
I am using above successfully when for whatever reason serial number for monitored disk has changed. For PerfCounter Custom I have a number of situations where its edit would save a lot of job. I mean I would change only minor things like disk number or SQL instance name and general query structure would stay the same. I do appreciate for such little feature.