Experimental Physics and
Industrial Control System

Looking at the docs, I would say the samplingMethod and the samplingPeriod properties (configured through the policy) should allow you to define how the engine gets a value.
https://slacmshankar.github.io/epicsarchiver_docs/api/org/epics/archiverappliance/mgmt/policy/ExecutePolicy.html

Additionally, the retrieval side should provide useful data for slowly updating PVs. If you ask the archive for a time span, the initial value for any PV should be the value of the last update before the start of the specified time span. Depending on the implementation, this may return time stamps that are outside of the asked-for interval, which is something that clients need to be prepared for. Needs to be specified properly.

Cheers,
~Ralph