I'm not convinced that the 5-10 ZIP Oyster card does count as an area-based concessionary pass. My view is that it is a PAYG product which just happens to discount your fare down to zero if you are within the London Zones, or down to 50% on selected journeys outside the London Zones. So I don't see how you can use it for a non-stop split.
Just my £0.02 though. Hopefully
@MikeWh can shed more light through his contact
It's very possible that it doesn't count as an area based pass, it seems we don't know. It obviously has PAYG capability but does it only operate as a £0.00 PAYG or does it contain a Travelcard?
It's been suggested that it's a Travelcard for bus/tram but not for trains. Does such a capability exist and why would it be configured as such?
The Zip 5-10 Oyster does not give 50% discount outside the London zones. That discount is only applicable to Uber boats. (Confusion with Zip 11-15 Oyster?)
So regardless of what unspecified and undocumented procedure you're meant to follow when venturing outside the London zones, the impact of doing so is nothing, the impact on TfL is nothing.
The only corner case I can think might be an issue is if a journey was starting outside the zones and coming inwards first. As we're Londoners were always going out so are always covered by the initial tap in, followed by the Boundary Zone ticket as we leave the zones.
On our return journey it's debatable whether the tap-in is still active and is closed by tapping out at our home station or whether that gets us a second "penalty" charge of £0.00 for the day. Whatever happens, it has no impact on the cost of £0.00 for travelling in the London zones, it doesn't appear to impact the PAYG balance of £0.00 and doesn't prevent the card from continuing to work.
Which is why I would encourage the OP to consider doing what we do. The free bit doesn't get paid for, the paid for bit does get paid for, nobody cares and life goes on.
However it's also worth looking at a Friends & Family Railcard option or Groupsave (does it exist on SWR?) depending on how many people are travelling.