I don't think there any ranking in normal mode. You have your level plus number of win/loss.
If order there is, it's definitely not a ELO score. (For normal game I mean)
Edit
I found this that may invalidate what I said:
The results of normal games count towards the Summoner's Win / Lose / Leaves score. Additionally, your ELO is changed according to the result, you earn points if you win and lose some if you lose. The number of ELO-points you gain or lose is calculated by an unknown formula, taking the expected result due to the ELO-values of all participants into consideration.
Source from : http://leagueoflegends.wikia.com/wiki/Normal_game
Idling in customs is probably not going to work, as your bot team will be 4v5. If the teams aren't even (say you have the enemy team 4 bots instead of 5, but your team still has 4, then you would get no experience).
Idling in games with normal people isn't cool, you will get reported and banned for wasting their time.
The only option I can think of is if you have a friend who can play the customs whilst you are afk. Even then you will have to move the mouse so you don't get kicked out of the games, and obviously you have to be present when joining games.
I really wouldn't rush getting a higher summoner level. You will be at a disadvantage to other level 20's who have played their games legit and will have a lot more experience than you. Runes also take a LOT of IP to save up for. You won't have a full rune set by the time you reach level 20, unless you use some IP boosters.
Games are meant to be fun, play with people the same level as you and enjoy yourself, don't worry about tier 3 runes yet.
Edit:
Ok so I didn't really consider the loss thing. If you are really hellbent on doing this then I will break it down for you.
Here is an excellent answer on IP/EXP gains per match: How much IP do you get for winning & losing games in the various modes/maps?
Custom games give 75% of max IP, losing gives around 70 IP per average game loss (40 mins), so thats about 52.5 IP per match.
This works out at 120 games (4800 minutes, or 80 hours) to get 6300 IP using this method.
The average PC has about a 400 watt PSU. For a gaming rig I'd say at least 600 watt. For the time your computer is on during these games, you have used about 48 kilowatts (600 watts * 80 hours). In the UK the average price of a kilowat is about £0.11. This works out to around £5.28 in electricity costs.
A 6300 IP champion costs 975 RP. You can buy 839 RP for £4.50. This means 975 RP comes to around £5.17.
It is actually cheaper to buy RP (although this doesn't buy you runes...)
Best Answer
Players whose name is displayed in red in the post-game score screen have already been flagged as AFK (which means they have been automatically disconnected, most of the time).
You can also report a player for being AFK if the server failed to notice. Players who run in circles at their fountain to bypass AFK detection can be flagged this way.
Players who have been flagged as AFK, be it automatically or manually, are then handled by LeaveBuster (NOT the Tribunal). See How exactly does Leave Buster work?.
There is no way to make sure they will be punished. You have to trust LeaveBuster for that.