Stop during HSS, or before you fire with a RS, and use outwit then attack?

No.

Outwit says it cannot be used during an action.

But don't just take my workd for it, from pg 3 of the FAQ that went into effect May 1st: (see last line)

OUTWIT: Replace the text with the following: (optional): Once during your turn, as a free action, this
character may counter a power on a target opposing character. Treat the target as if it does not have the
countered power. That power remains countered until the beginning of you next turn. This character must
be within 10 squares of the target and have a clear line of fire to the target. Instead of a power, you may
choose to counter an opposing flying character’s ability to soar; in this case, lower the soaring character to
hovering mode. If this character loses Outwit or is KOd, the countered power returns immediately. This
power may not be used during another action.

tyro is, for now, correct.

However, once the Universe rule set becomes official (as of June 2), this may change depending on the FAQs at that time.