public key on R/O device? (trustdb unwritable)

[elf@florence.buici.com]

I want to put a public key on a read-only device and use it, along
with gpg, to perform a signature check.  

  gpg --no-greeting --batch --verify --homedir /floppy - 

While the signature verification succeeds, this command returns a
failure code when the floppy is read-only because the trustdb cannot
be written.  The return code is 2 which appears to be the same result
as when the public key cannot be found.

Is there an override that will prevent updates to the trust database?