Yeah, at the moment I shoot scdaemon with SIGTERM whenever I need to use the PIV
app, which is rare, and have carefully avoided any kind of automated invocation
of the smartcard through scdaemon (e.g. my statusbar polls via ykinfo directly,
rather than invoking gpg --card-status.)

I know essentially nothing about smart cards or PC/SC's design, but what goes
wrong holding the card open shared rather than exclusively? Can other shared
lock holders do drastic things like insert or remove keys, causing scdaemon's
cache to become stale? I would have (naively) guessed that shared holders could
only do things like cryptographic operations which won't pose an issue to
scdaemon's cache. (Admittedly, cryptography is not side-effect free; counters
get incremented, random numbers get generated, but none of that is the kind of
thing that scdaemon caches, right?)

Thanks for thinking about this. :)