Send/Receive Procedures in pppd 2.4.1

I have a Multitech modem that will support AT commands even while the
PPP link is up if I send this:

<wait 2 seconds> +++ <wait 2 seconds>

.... and come back online when "ato0" is issued.

I'd like to do this while the PPP link is up because the modem is
aware of certain things. It's wireless, and knows the signal strength
it's seeing, the VSWR, and so on. I'd like the Linux box to know
these things, too, so asking the modem seems natural enough. It would
also permit SMS messaging, which the modem can handle through AT
commands alone.

After chasing down several dead ends, it looks like I'm going to have
to put procedures to handle this in pppd itself (since pppd doesn't
look like it wants to share the serial port; even the usual read(,,)
and write(,,) procedures fail). What are the procedures in pppd that
send and receive characters? And what procedures are run when the PPP
link is considered "up"?

Thanks in advance. I'm rather new at this.