After some suggestions from Jan, and then some other work to get the syntax for SDCC's variable attributes (which conflicted with the preprocessor I was using), I managed to get a workaround functioning -- Thanks!

Instead of using (* (uint8_t __xdata *)0xDF53, which appears to cause some error, I declared a variable with

volatile __xdata __at (0xDF53) uint8_t RXFIFOCNT;

Now it compiles without error.