Can I use a pull-up resistor to raise Voh beyond 3.3V?
Yes, you may do this. There are a couple of items to keep in mind:
1. There will be current sinking into the 3.3V supply from the 5V supply when the XPLA3 device is driving high.
2. The transition from 0 to 3.3V will happen quickly, and the move from 3.3V to 5V will happen slower, depending upon the value of the resistor and the capacitance of the node. Be aware of timing issues.
Please see (Xilinx Solution 6717) for an alternative method of driving 5V out of a 3.3V device.