Re: Relocating to SoCal to liveaboard - have a catch 22...
I think your first step should be phone calls to every marina you can find a number for, and see if they have a slip that size, or what the waiting list is, maybe don't even bring up the LAB topic. Finding a big enough slip is probably your first hoop to jump through. I doubt you'll find one in MDR, more likely in San Diego bay somewhere, or Long Beach. Once you've narrowed it down to availability, those should be the ones you go visit, show them pictures (for sale ads) of the boat(s) you would buy, if they consider you. This let's them know you're doing your homework and they can rely on you to commit.
The other option is buy the 40'+ sailboat you want where the for sale ad mentions that the slip is transferable, so it takes care of all the hard work of finding a slip. These are rare, but if you create a good relationship with the current owner of the boat, and go see it when you're in town, and speak to the dock master, then you'd probably be welcomed to take it over. The nice thing is sailboats that size stay on the market a long time these days, so you can be patient.
When I was going inside, face-to-face with marina managers/dock masters, in my 10 minute conversations alone, there would be 2 or 3 phone calls interupting the manager, all asking about liveaboard slips in a big size. The manager just says "no" to get them off the phone. Truth is, some can be flexible IF they want to. Privately owned marina's can allow more LABs, but the general rule is to at least keep every LAB with one 'empty' boat between them, so the bigger or in some cases "less popular" (for various reasons) the marina, the better chance you may have.
Good luck! There's been a gulfstar 44 in MDR on the market for almost a year now, FYI