@sonusfaber
i see 3 kind of solutions
you can create a normal datapoint with your adapter with permissions only read
you can safe your information in the object data of a datapoint, maybe in the datapoint that represence your instance (adapter.0)
for example the history adapter safes his data with the object path common.custom.history.0 and the sql adapter safes the data in common.custom.sql.0
to see this in real life go to a datapoint, enable the logging with the history/and/or the sql-adapter, open the datapoint setting with the pen icon beside and select the object data tab.
you can safe your data as a file below the following path
/opt/iobroker/data-files/(name of your adapter)
my adapter tvprogram, the history and the backup adapter also safe also some data here