I'm trying to make a scene with a few small planets, but when I went to make a lake/river I couldn't find any options to have the fluid's gravity be relative to a point (aka the planet's center). I've tried putting a forcefield at the center of my planet but the fluids just seem to ignore it completely even if it works for rigidbody objects. Would it even be practical to do my lakes this way, or would it be better if I made a mesh to fill the lake instead of bothering with fluid simulation?
You could use particles set to 'Fluid' type and rendered using a Metaball as the dupli-object. While this won't produce a full fluid simulation it might be good enough for your usage. Using particles means that the 'fluid' will be affected by the force fields (the true fluid simulation is not affected by such force fields) so you can disable gravity and use a force to draw the 'fluid' to the planet.
This can produce the following :