Does the Sakura (or any other boards) have a built-in IEBus driver/receiver? There doesn't seem to be much documentation about using the Sakura with CAN or IEBus i/o. The RX63 itself doesn't have one based on the documentation.
Looking at the sheet for the R5F563NBDDFP, the IEBus ports are located at pins 29&30, P17&P16. Those two pins are broken out to the Sakura board connector CON8. On the bottom of the Sakura board you can find the designations for P17 and P16, but in the bootloader I don't believe they are specifically configured to be recognized as IETXD or IERXD.