ralphot - Air doesn't flow into a cartridge. The indentations on the sides of the carts provide the air flow bringing out the vapor through the mouthpiece. The wicking action is caused by the filler material sitting on top of the metal mesh on the bridge inside the atomizer along with aramid fibers which help to hold some liquid which create some flow into the pot below the nichrome wire and Aramid fibers in the coil of nichrome that go to the pot that heats above the liquid in the pot.
http://www.e-cigarette-forum.com/fo...-suppliers-whats-my-atomizer-5.html#post41087
And a great thread with pics of a 510 atty disection:
http://www.e-cigarette-forum.com/forum/modders-forum/48467-510-atty-disassembly.html
That's where you get your vapor. If your material doesn't wick right, you won't get much vapor (not releasing it or flooding your atty). If the material in the cartridge could heat up... you'd melt the cartridge. The liquid actually helps keep a cooler temperature, but optimal for vaporization.
Cage - just about EVERYTHING you can image has been tried (and if you search the forum, you'll find a zillion threads).
The stainless/steel wool and mesh (similar to what is in a bridge in an atomizer, but what is in the atomizer is nickel foam) will give you a nasty flavor to your eliquid (not right away, but guaranteed it will) as well as any other brass, etc. metal mesh. It doesn't hold on to liquid well and floods your attys, is brittle and breaks off in little bits and travels inside your atty. Many have tried it and several reported a gunky residue inside their atomizer after using it a while (chemical reaction with the metal and liquid? don't really know). There are so many other alternatives that have been tried and work (see my earlier post and links) that are inexpensive and others have used it for well over a year so it's safe.
Experiment if you want, none of it is anything new. Some of the other tried materials tried on the forum since early 2008 are sponge, human hair (yeah, FAIL... went to the Not Recommended section of the forum), natural lamb's wool, cotton, cheese cloth, paper napkins, coffee filters, toilet paper, Sham Wow, roll your own cigarette filters, rayon and nylon underwear

, lollypop sticks, paper towels, tea bags, twine, gauze (rayon/polyester), plastic pot scrubbers, Buff Puff facial scrubbers (has a binding agent in it, so I wouldn't ever try it but people used it with reported success), and the list of weird stuff goes on.
What works that I've read about and tried with success (other than fish filter medium which we know is safe) is Lipton Pyramid Tea Bags (known as PTB, which is a PET plastic and many had decent success with it in the small 510 carts but is a pain to roll right to fit and wick right), Rayon or Nylon ribbon (similar to the Lipton Tea bags), Straw modification (small coffee straw added to the side of the regular fiberfill found in carts, helps get air to bottom of cart to end airlock holding in juice and works well in all models)
http://www.e-cigarette-forum.com/forum/tips-tricks/11421-cart-mod-better-refilling.html, small spring modification with polyfill (called ADP mod. Here's the thread:
http://www.e-cigarette-forum.com/forum/experiments-equipment/46125-i-call-adp-mod-auto-drip-mod.html) and ZFM (zero filler mod)
http://www.e-cigarette-forum.com/fo...supplier-forum/53487-zfm-zero-filler-mod.html
See most of the ideas for cartridge fillers and modifications are here in this section of the forum (27 pages of threads on cartridge fillers and mods!)
Cartridge Mods