What exactly are the tradeoffs between terminating a transmission line carrying a digital signal in 50 ohms versus high impedance?
On many electronic designs transmission line effects do not need to be considered until about 40MHz or 50MHz.
For a 1.28MHz signal, a transmission line is most likely not needed. The wavelength is over 200m so unless the cabling system is on that scale, reflections and ringing are not likely to be noticeable. This means if you use high impedance as a clock load it will be fine.
However, if you are running the clock to multiple sources fanout will need to be considered, as if the total impedance (resistive load and capacitive load) is too low the clock driver will not be able to provide sufficient current to raise the voltage to the high level. So check the fanout and use a sufficient driver for the load.
If transmission line effects are not negligible, then a wave on a high impedance load could reflect back to the source. In this case the reflection coefficient should be calculated and the load adjusted.