Ecash mints need to periodically rotate the private keys they use to sign ecash so every ecash opendime would have an expiration date. Also, consider you are loading custodial bitcoin IOUs onto hardware, so there's no guarantee it will still be valid when you go to spend. There is also no way to audit the ecash on the device, so you have to spend it to validate it. I think the tech is just not suited for this use case. Better to load bitcoin onto physical hardware: it never expires, is trustless, non-custodial, easily audited.