Can I get a visa issued so far in advance of my entering PRC? I cannot see how I can get one while on my travels.

The embassy usually suggests that the applicants submit their applications no earlier than three months (one month in advance before departure is better). And the single entry L type is usually valid for three months. So I think you might apply for a double entry L (with the validity of six months) in advance. But I am not sure if you can get it because the consular officer has the final say.