This is the method to get additional mods you usually wouldn't be able to get in LS Customs. Like wheels on the Sanchez 1, turbo on the Voltec and a lot of mods on the Dune Buggy.
*NOTE that you don't have to plug out your internet cable on PS3, simply sign out of PSN from the PSN-menu.
This is confirmed working after patch 1.07
Here's my Sanchez with modded wheels (and the emblem for some reason):