I don't think you CAN do it externally. I've tried various memory editors, and none of them did anything other than produce a horrible wrenching effect on the player (jerking back and forth between the fake and real position/velocity).
I think you would be able to do it internally if you wrote your own Blockland client, but who's actually going to write a whole new client for Blockland when it can both get your key revoked and is just not worth the reward?