Here are a series of commands that will allow you to /execute relative to a specific block. (Tested in 14w21b)
Remember to leave a Diamond to show your support!
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/scoreboard objectives add BlockExecute dummy

/fill ~7 ~-1 ~ ~ ~-1 ~ redstone_block
/fill ~7 ~1 ~ ~ ~1 ~ <bedrock>

/execute @a ~-2 ~-2 ~-2 /fill ~ ~ ~ ~4 ~4 ~4 double_plant 0 replace
/execute @a ~-4 ~-4 ~-4 /fill ~ ~ ~ ~8 ~8 ~8 double_plant 0 replace

/scoreboard players set @e[type=Item] BlockExecute 1 {Age:0s,Item:{id:minecraft:double_plant}}

/execute @e[score_BlockExecute=1,score_BlockExecute_min=1] ~ ~ ~ <COMMAND(S)> 

(optional) /execute @e[score_BlockExecute=1,score_BlockExecute_min=1] ~ ~ ~ /setblock ~ ~ ~ <BLOCK>

/kill @e[score_BlockExecute=1,score_BlockExecute_min=1]

/scoreboard players reset * BlockExecute
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EXPLAINATION:
Relative to the player, a /fill replace command is executed. This replaces all of the blocks of the player's choosing with a "double_plant" block. When updated, the double_plant block will break into an item and from this entity we execute our commands.
After the commands have been executed the double_plant item is destroyed and the scoreboard is reset.


Special thanks to dandamannnnn for his Destroy Specific Blocks command set up.
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