Jeppe, just to get the task accomplished this is what I would do.
- Run a report looking for the configuration line
- Export to excel the result of the report
- Delete anything that the rule did not match from excel
- use the formula ="UPDATE Nodes SET Your_Custom_Property = Whatever WHERE Caption = '" & A1 & "'"
- Fill down
- Paste the info into a sql query and run it
I do this quite often on one-off things I just need done quickly. You know something else I just thought of, you could use a remediation script that change one of the default SNMP values of the device. For instance, If XXXX is not found then set location to WHATEVER. Then you could use the location field as your "custom property" because then you can just rediscover the device and it will autopopulate. . Hope this helps some.