If you want to try an approach that does not require an external script, you could try creating a parallel process event and enter the following script call in the 'Conditional Branch':
$game_party.members[0] == $game_actors[x]
where '0' is the position of an actor in the party (in this case, the party leader), and 'x' is the location of the actor in the database.
this could work... but need some change to be able to used for event condition
set one common event. make it run at
parallel process
in the common event page
use change variable event command > script:
$game_party.leader.idadd wait command (30 or 60) so it won't be lag
it will store the party leader id to variable you choose.
then in event huge boulder. use the variable condition (below the switch condition)... which will activate when that variable you set in common
event equal to certain value. done without script

. so you can create huge boulder that can be only pushed by certain member in leader position.
edit: sorry but i think i'm wrong. variable event condition only support variable equal to x or
above. i think it's like the conditional branch before

. silly me.