I wouldn't touch the idle of the engine to solve an idle boom.Wow, same issue. Once I hit 1,000 rpm the noise dissipates. If the idle is turned up would it solve this?
Grab a measurement mic with an RTA (or find a friend with one, or get a subwoofer test cd) and find out what frequency the boom is.
Toyota made 5 different weights to cover 5 different frequencies from 32 hz up to 48 hz. Find your booming frequency, install weight, problem gone (or so that's the theory).
For the LX, the TSB is EG012-05. Not sure on the bulletin number for the LC.
I also suspect some high density sound deadening (dynamat, noico, etc...) of the floor or roof would solve this, too. There's a lot of flat, large steel panels on the 100.