Newbie question: Device permisssions

[Alec Leamas]

Dear list,

I'm trying to package opencpn, a navigation program requiring access to
serial data, typically on a device like /dev/ttyS0 or /dev/ttyUSB0.

In traditional rpm packaging, access to these devices is handled using
the group permissions. The devices are typically 660 and owned by
root:dialout. Adding dialout to the running user's secondary groups
gives her r/w access to the device without too much fuzz.

Howwever, my too small brain cannot understand how to handle this is in
a flatpak package. The device ownership in the sandbox is
nfsnobody:nfsnobody, so the sandbox user cannot access it, permissions
being 660.

The brute force method would be to add a udev rule changing permissions
and/or ownership of the device. But, before making this somewhat drastic
step: Is there any other way to give the sandbox user r/w permissions to
the devices involved?

Cheers!
--alec