Hmm, lets see. The melting point of lead is fairly low, 327.46 °C and it is a good absorber of radioactivity.
I think the problem with the reactors is not the heat per se, but the exposure of the fuel rods to the air without cooling because of escaping steam not being replenished by cooling water. The remaining steam etc may blow out the thick container and radioactive material will disperse some distance while the fuel rods will melt.
I expect that the design is such that even in melt the fuel rods and melted control rods will be sub critical.
The problem with the suggestion is, when they have troubles pouring cool seawater in, how ever will they be able to pour hot liquid lead? It is not that the reactor is a pot whose cover can come out!
A better question/suggestion would be, since lead has such a low melting point, and the operating temperature of a boiling water reactor is 250C, why don't they set up a sort of cladding to be passively used in such emergencies: the inner layer of something that can melt when temperatures get higher than 500C, and then lead melts and floods out the system before the fuel rods get exposed, covering them. The answer is similar to why they were not pouring seawater on all reactors from the first minute: economics, trying to save the reactors for production later.