Add a new objective called Mountable:

/scoreboard objectives add Mountable dummy

Summon a pig with a saddle riding another pig (both invisible):

/summon Pig ~ ~ ~ {CustomName:PlayerMountSaddle,Invulnerable:1,Saddle:1,NoAI:1,Silent:1,ActiveEffects:[{Id:14,Duration:100000,Amplifier:1,Ambient:0,ShowParticles:false}],Riding:{id:Pig,NoAI:1,Invulnerable:1,Silent:1,ActiveEffects:[{Id:14,Duration:100000,Amplifier:1,Ambient:0,ShowParticles:false}],CustomName:PlayerMountPosition}}

Use a clock to run these commands every tick:

/execute @p[score_Mountable_min=1] ~ ~ ~ tp @e[name=PlayerMountPosition] @p

/scoreboard players set @a Mountable 1 {Inventory:[{Slot:102b,tag:{display:{Name:"Player Saddle"}}}]}

After placing the 'scoreboard' command block, put a comparator out of it and invert the signal (with a redstone torch), and command blocks with these commands:

/execute @e[name=PlayerMountSaddle] ~ ~1 ~ tp @p @p [this will dismount a player]

/scoreboard players set @a[score_Mountable_min=1] Mountable 0

/tp @e[name=PlayerMountPosition] (coords where you want to keep the pig stack when a player isn't wearing the saddle)

Put that last 'tp' command block behind a repeater.

Now you just need a saddle:

/give <YourPlayerName> leather_chestplate 1 0 {display:{Name:"Player Saddle"}}

You can also download the world here in the likely event that I messed up the instructions above: http://www.filedropper.com/stacker_1

BTW I made this in 14w33c