At the moment, the majority of my hair is Cupcake Pink. I was thinking that next I'd like to either:

a) Go blonde.

or

b) Dye most of my hair a very blue-toned purple. (I was thinking either diluted SFX Wildflower or Directions Lilac.)

But either way, I'd need to get the pink out of my hair.

So my question is, would it be possible, once the pink is a pale pastel, to tone it out of my hair with a pale green?

Logically, it should theoretically work if you can tone out yellow with lavender and orange with blue, but I'd personally rather not take the risk with my whole head without consulting other people. I was thinking of probably diluting either SFX Sonic Green or Iguana with a massive amount of conditioner in order to make the green toner.

So what do you think? Will it work or will it just turn into some terrible mess?

It might end up just turning it yucky brown.

I think the green toner idea is going to be really hard to get right without it looking mucky. The pink would have to be really really light. The same happens if you have yellow hair and you use a darker purple than a toner it goes a weird brownish blonde.
Since you want lilac, how about applying a very watered down blue over the pink when it fades out a bit more?