Surprised you've gotten no responses yet... I can't give you too much hard info but it sounds like what JTB does is similar to what a lot of shopping services will do, or even some credit card companies (AMEX will do it if you have the right card).
This site has some info on the Lawson/Pia process: http://www.tokyogigguide.com/tickets
You need the event code, which you can get from Scandal's web site if tickets are still available. The date you can get tickets from Lawson/Pia depends on the event - for regular concerts it seems to be about a month in advance. Scandal post the sale dates on their site also. Larger events usually have several different levels of ticket release, going from the best seats outwards, and they go on sale on different dates over the course of 3-4 weeks, with general admission still going on sale about a month before. There may not be many seats left by that point, however.
You can sometimes get tickets from the advance sale pool if you try - I got tickets to Budokan from the second level of advance release - I can't remember what they call it, but this may be the "raffle" thing you're talking about. I had to fill out a form on their site and wait for their email to say I "won". I believe they sold *most* of the tickets to that show this way, since our seats weren't really that good (2nd level on the side). You need someone who can help you with Japanese for that, though, and then you still need someone to actually pick up the tickets at Lawson (JTB could probably do that). Otherwise, for an event like that you probably wouldn't have been able to get tickets because the general sale tickets sold out in 15 minutes, and JTB probably isn't going to be standing at the machine waiting for you at the exact time tickets go on sale. A shopping service might, though.
A more regular show should be easier because anyone can just show up at Lawson and buy tickets, but you would still want to make sure they do it on the date they go on sale since Scandal usually sells out fast. You may even want to give them event codes for several different shows you could make it to, just in case.
I would just call JTB and see what kind of service they actually offer.