Proposal: A Place for your Things
In the rule “Heists {I}” add a subrule named “Stashes {M}” with the following text:
Each Participant has a publicly tracked Stash, which is a list of zero to two alphabetical characters, defaulting to zero characters.
As a Heist Action, a Participant may remove one alphabetical character from a word in a Mutable rule and add it to their Stash, as long as the word that was modified is a word in the English language after the modification $$.
As a Heist Action, a Participant with an non-empty Stash may remove any number of alphabetical characters from their Stash and add those characters to a word in a Mutable rule, as long as the word that was modified is a word in the English language after the modification $$.
Trying another way to be able to manipulate multiple characters at once. To be able to add 2 characters at once will cost 3 Heist Actions overall, but the advantage is being able to save “good” characters for later when you need them.
This post is still within the 
ais523: Mastermind
This seems fine to me.