Powdered Nose

[I am the brainwasher]

Let's draw the most fictional scenario, propably like ever, that a player shows his Serum Powder in his opener in order to remove his hand to draw a fresh new 7.
Then the opposing player (tries to) remove Elvish Spirit Guide and Simian Spirit Guide to cast Manamorphose for double blue Mana to Stifle the Serum Powder's ability.
I am pretty sure that this is not only completely dumb but also impossible due to the fact that the game hasn't startet at that point, but just wanted to make sure thats correct.
Plus, what is Serum Powder's ability then in terms of ruling?
Is it a replacement effect? An activated or triggered ability? Does that use the stack (which it couldn't because the game hasn't started yet IMO)?
Thanks in advance.

[Julian23]

In order to cast Stifle, thou will need priority. No player receives priority before the Upkeep of the very first turn. Therefor, no stifling of Serum Powder. Also, Serum Powder's mulligan-related ability is neither activated nor triggered. Therefor not even a legal target.

[I am the brainwasher]

That's what I was almost sure about, but I am still confused how that (let's call it ability) is defined in the rule book, under which part it falls.
Thanks though!

[cdr]

See rule 103 (http://yawgatog.com/resources/magic-rules/#R103).

103.4a. If an effect allows a player to perform an action "any time [that player] could mulligan," the player may perform that action at a time he or she would declare whether or not he or she will take a mulligan. This need not be in the first round of mulligans. Other players may have already made their mulligan declarations by the time the player has the option to perform this action. If the player performs the action, he or she then declares whether or not he or she will take a mulligan.

Between drawing hands and "opening hand" abilities like Leylines.

[I am the brainwasher]

Thank you!