The datasheet says
9.6 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8.0MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user.
I haven't tried it, but that statement makes me think it might be possible, assuming voltage and temperature are relatively steady, and the equipment and time to calibrate the internal oscillator are not issues. Calibration is specific to each individual part, so if this is not a one-off, then that is also a consideration. Seems I saw references to some software or firmware to aid in the calibration, so I might Google around for that. So definitely some experimentation is in order.
But I have to ask, how much space is being saved? Even though an HC49/US crystal is fairly small, separate load caps can be eliminated by using a resonator which is still good enough for serial communication.