I have an apparently healthy lemon tree that exhibits a few isolated leafless branches, 10-30cm long. Can I assume these are dead wood and prune them, or should I wait longer for them to emit new leaves?
I would scratch off a small piece of bark. If the cambium layer below the bark if brown that limb is dead. If it is green it is alive. Start at the very end of the limb. Scratch off some bark. If that part is brown trim a few inches off. Working your way back to the main trunk. Part of the limb might be dead, but other parts are alive.
Lemon trees will defoliate under stress. They do have the ability to regrow new leaves, but first you must fix the problem. What happened that made those limbs to defoliate. Check for insects and mites. Look for signs of disease like spots on the limbs. Check you watering habits. Make sure you only water after the soil is mostly dry. Lemons require very well draining soil. Some people use Cactus & Succulent soil for their citrus.
Once you have identified the problem and made the correction then those parts that are still green will probably releaf.