PLL outputs to drive 50 ohm loads

You can (almost) always use serial termination for single-ended point to point connections. I overlooked FvM's observation that unfortunately you can not apply series terminations for 3.3V outputs. I suggest you add a series resistor externally. The trick here is to estimate what the series resistance of the driver inside the FPGA is, I usually specify a 39 ohm external resistor for a 50 ohm Z0. A 0402 resistor can fit directly to the BGA escape via. Also the transmission line you route on the PCB doesn't have to be a 50 ohm either, you could opt to route for 60 ohm (or even more).