Wring out those hankies, please, and stop wailing "Goodbye!" to exiting French Ambassador Francois de Laboulaye. Simply sniff "au 'voir." He's planning to bounce right back at cherry-blossom time, into a chic little pied-a-terre in Georgetown. Le tout Embassy Row naughtily calls this Being Your Own Ghost, or Pulling a Peter Jay. (That's after the dear departed British Ambassador who didn't depart.) When this happens, you see, everybody keeps muddling up the old man with the new man. The completely wrong Ambassador gets asked to parties for months, 'til they straighten out the Green Book. Ear, personally, thinks this livens things up nicely . . . Meanwhile, down the Row apiece, they're bracing for a brand-new British Ambassador, this summer. Sir John Thomson is the much-bandied name. The Row's old free-loaders fondly recall him in his happy pre-Sir stage. He was First Secretary at the Embassy here in the early '60s. He's Ambassadoring away in India right now. There is absolutely nothing wrong with Sir Nico Henderson, the present garden-loving Brit. It's just that he was pulled out of Retirement Mothballs to serve by Maggie Thatcher, and Time Maches On, and the Rumors are Everywhere. Stay tuned for late developments.