Hey Wilflower. There are several products on the market that do exactly what you are describing - they remove the salt buildup. I would suggest using Clearex

Here is a link that might be useful: Clearex

Just a thought but are you leeching with pure water? Many municipal and well waters have a good deal of minerals in them and could even cause buildups. DI or deionized water is extremely hungry if it is available to you and should leech best. If not try catching rain water to leech with as the cheapest source of the purest water available to you.

The pebbles might retain less moisture but you need to consider your area and the increased chance of roots drying. You could also experiment with systems without media  aeroponics, DFT, or NFT. There is a commercial low desert grower near me who uses only NFT. Low desert hydro is supposed to be more difficult, but they are successful with this system.

Maybe this will help with media:
http://faq.gardenweb.com/faq/lists/hydro/2004060437029193.html

Hello Wilflower,
Do not use hytdroton from the baseball field, as it has a PH problem. The hydroton for hydroponics has a neutral PH.
Of course they'll sell it for a higher price....
I use it with my hydroponic tomatoes.

I know everyone says this is really expensive, but I'm pretty cheap and I use it...'course, I have a teeny system. I bought a big for under $20 (substantially under, I think, but I don't remember) that filled my 10 pots and had at least half left over, and it's reusable. I guess it depends on the size of your system. You did say *large,* didn't you. :-)