I made an modified version of this extension.
In addition to what it normally can do, you can now put a REMOVE at the end of an effect. If you then activate that effect, it will be added to the token as normal, but without the REMOVE. The REMOVE part is then executed (i.e. removing what you wanted).
Example:
Effect 1) "Hidden; ADVATK; GRANTDISATK" (write without quotes, and set targeting to 'Self' and expire to 'never')
Effect 2) "Sneak attack damage; DMG:1d6; REMOVE: Hidden; ADVATK; GRANTDISATK" (write without quotes, and set targeting to 'Self' and expire to 'expend on next roll')
a) Player rolls Hide skill, and then click 'Effect 1' - Hidden i applied to the character/token
b) Player attacks with sneak attack (gets advantage from 'Hidden' effect)
c) Player hits, and then clicks 'Effect 2' to indicate that he will get sneak damage on next damage roll. At this point 'Effect 2' automatically removed 'Hidden' and applied 'Sneak damage' (Note: if player missed, he would just click 'Effect 1' again to auto remove it)
d) Player rolls damage (and auto gets sneak damage also). 'Effect 2' is auto removed (as part of 'expend on next roll')
Here is the modified extension:
Attachment 25561