Is there a cycling equivalent to Naismith's rule
As you have noted, the problem is slightly more complicated for a bicycle since aerodynamic drag is a larger component. However, one can combine two rules of thumb which are given in these two bicycles.stackexchange answers (How do I calculate power to climb a hill and How many miles of riding are equivalent to one mile of running) to make an estimate of either speed or power on the flat and on climbs. 

If you are only concerned about climbing the rule of thumb is simpler: on a steep hill, multiply the hill's gradient by your speed in km/h, then by ~ 3. If you measure your speed in mph, multiply by 5 rather than 3. That will give you a ballpark estimate of the watts/kg you need to produce. Since you're trying to calculate riding speed, just "solve backwards" given the equivalent power (in watts/kg) for a given slope. For flat roads where aerodynamic forces predominate, you will need to have a ballpark estimate of your "drag area" which mostly depends on your position on your bike.