With a vegetable peeler, pare skin from squash and discard; cut squash in half and scoop out and discard seeds. Cut squash into matchstick-size pieces (1/4 in. by 1/4 in. by 2 in.).
Rinse lemon grass and pull off tough outer layers. Cut off stem ends and coarse leaves. Cut stalks into 1-inch pieces.
In a 1-quart glass measure, stir together lemon grass, vinegar, sugar, and peppercorns. Heat in a microwave oven at full power (100%) until mixture boils, 3 to 4 minutes total. At once, add squash, and let stand until tender-crisp to bite, about 15 minutes.