Can Mew's "Lost Link" Poke-BODY use the attack of one half of a LEGEND Pokemon if its card is in the Lost Zone? Or what if both halves of the LEGEND Pokemon are in the Lost Zone?
No, you cannot. If a LEGEND Pokemon's cards are not in play, you cannot reference its attacks, Poke-POWER, Poke-BODY, etc.
Source: Triumphant FAQ (2010-11-04), PUI Rules Team (2010-11-04)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_97