Hi Jonathan,
I created a F28335 project and checked; both Semaphore_pend and Mailbox_pend are using the xdc_UInt32 type for the timeout value. Please see the API documentation:
Mailbox_pend: http://software-dl.ti.com/dsps/dsps_public_sw/sdo_sb/targetcontent/bios/sysbios/6_40_01_15/exports/bios_6_40_01_15/docs/cdoc/ti/sysbios/knl/Mailbox.html#pend
Semaphore_pend: http://software-dl.ti.com/dsps/dsps_public_sw/sdo_sb/targetcontent/bios/sysbios/6_40_01_15/exports/bios_6_40_01_15/docs/cdoc/ti/sysbios/knl/Semaphore.html#pend
You are correct, xdc_UInt is a 16-bit value, so if you assign BIOS_WAIT_FOREVER to a xdc_UInt variable it will be equivalent to 65535. If you would like to use a larger pend value, you have to either use the BIOS_WAIT_FOREVER macro directly, or pass in a xdc_ULong variable.
Regards,
--Emmanuel