If you want to avoid the work of programming the SCC (Serial Communications Controller) in your IIgs directly, you want to use the Pascal entry points in the firmware. The SCC is compatible with the SSC (Super Serial Card) at the firmware level only. They are completely different pieces of hardware.
But you've got the right idea already: you load up command strings and knock on the entry points to do things like set the baud rate. As for PR#2, I suppose a simple JSR $C200 would do the job. Check your references for "Apple Pascal 1.1 Entry Points" or so. I have some ADT source code from Joseph Oswald that implements Pascal entry points to do the things that normally happens at the hardware level so it could be made compatible with the IIgs SCC. Send me a PM if you'd like to take a look at that.
But you've got the right idea already: you load up command strings and knock on the entry points to do things like set the baud rate. As for PR#2, I suppose a simple JSR $C200 would do the job. Check your references for "Apple Pascal 1.1 Entry Points" or so. I have some ADT source code from Joseph Oswald that implements Pascal entry points to do the things that normally happens at the hardware level so it could be made compatible with the IIgs SCC. Send me a PM if you'd like to take a look at that.