I know of people who have done this without being caught....but if they catch you, they bill your credit card the same price for luggage transfer that they would have for a person--$25. As far as I know, they don't have a "luggage only" transfer.
Please, I do realize that you asked how to do it "legally." I'm not trying to imply that you would do it on the sly.
I do have a question though. IF you are flying one of the airlines that has port check in--aren't you really transferring it to the airline at that time? And if that's true,
DCL isn't transferring it, the airline is? So, in that case, doesn't it make sense that anyone can do it? I know that the personnel at the port who take the luggage are airline people, not DCL people. I do realize that not all airlines have check in at the port available.
Could you maybe call the limo company and ask them to be sure that the vehicle they send is big enough for all the luggage?