Brasserie de l'Institut Franco-Japonais ^ ^ ^

Especially since it is a historically significant location for you, I think it would be pretty hard to beat. Can you wait a few weeks until the weather warms up? Then you could reserve a table outside for lunch.

Other ideas? Maybe leave the city altogether - go to the Fuji five lakes area (Climb up the hills behind Lake Motosu & Lake Shoji and propose at the top with the spectacular view ... )