The basic question is this: can you respond to the declaration of heedless revelry (for its second effect)? Here are the relevant texts:
Heedless Revelry wrote:Alternatively, may be revealed as an on-guard card on a company after the successful play of an item, ally, or faction. Tap all untapped non-Ringwraith, non-Wizard characters in the company.
CRF wrote:A revealed on-guard card retroactively takes effect as though it were both declared and resolved immediately prior to the chain of effects during which it was revealed.
In my view, you generally have the option to respond to an action if it's your turn. Therefore, you should have the option to respond to Heedless, all things being equal. But things aren't equal: the only problem is that CRF rule, which says the card "retroactively takes effect as though it were both declared and resolved immediately prior to the chain of effects during which it was revealed." Without the CRF entry, the story would then run as follows:
Heedless is on guard.
(1) The company successfully plays an item. (this is actually the end of a chain)
Response:
(2) Heedless is revealed.
(3) Response to heedless.
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(3`) The response to heedless resolves.
(2`) Heedless resolves, tapping all untapped characters.
With the CRF entry taken into account, we have the following:
Heedless is on guard.
(a) Heedless is declared.
(a`) Heedless resolves, tapping all untapped characters.
(b) The company successfully plays an item.
Make sense?