Will the new ldisc mess with the tty stuff to prevent "normal" serial
data and line settings from being handled properly?  If not, that all
looks very good to me, thanks for working this out.

It'll have no effect on that. What it will do is provide a way for things
like ISO7816 ldiscs to use the extra pins (as they can call from the
ldisc into the gpio layer) and to know the mapping of the extra control
wires, which varies by device.

Alan