This is the official page for information for passengers on the ongoing industrial action.
https://www.nationalrail.co.uk/service_disruptions/industrialaction.aspx
Details are currently only published for earlier dates but there is no reason to assume it won’t be exactly the same ticket acceptance for future strike days.
It is quite clear that tickets for the strike days will be valid on the day/s before and after. There is no exceptions or caveats given, your interrail is just as valid as any other ticket for this rule.
Under no circumstances should you add the replacement travel day to either a mobile or paper pass if you will also need to use the pass on the strike day.
Enter the strike day on your pass and enter the equivalent of the trains you take for that day, if challenged explain that to add the correct day will mean using an additional date and you are availing of the agreement to allow tickets for strike days to be used on the day before.
I would say it is unlikely you will have any issues with this, the staff are in dispute with the Government and their employers not their passengers so it is not in their interest to harass passengers over strike day tickets.