I was wondering the same, I noticed you can't use it once you do this. I was trying to share my car with a friend and accepted I could not use it once shared, but he could not see it to use it.
Were you able to get this to work? A couple of my PSN friends were trying this last night in my lobby and no matter what we did, it wouldn't work. I set two of my cars to online status but when they were in my lounge, they never had the 'online' tab for my cars and I didn't have one for theirs.
Maybe a new subject, but can you assure that a car is available in the MyLounge.
You can restrict the cars you can use in a race in MyLounge, but the system seems to select a list of appropriate cars for this, not including your favourites.