AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

#include <stdarg.h>

extern void putchar();

int
vprintf(fmt, ap)
	char *fmt;
	va_list ap;
{
	return(_doprnt(fmt, ap, &putchar));
}