The Black Riders Rulebook states:

"...that player may exhaust any number of characters he controls to commit those characters to the Hide test."

The Core Rules state:

"Characters committed to a quest are considered committed to that quest through the end of the quest phase."

If a character commits to a quest, is then readied (fast hitch, unexpected courage), and a revealed card initiates a hide test, can they also be committed to the hide test? I believe the answer is 'no'. However, the wording in the Black Riders rules is a little weak. There is nothing definitively excluding such characters from participating. Perhaps they retain their "committed to the quest status" despite assisting in the test?

A tangent on this idea would be a card such as Ranger Bow. If a character was committed to the quest and then readied, could the character and ranger bow be exhausted to deal one damage prior to quest resolution?

Is there a rules clarification that I missed somewhere that settles this issue?

You can quest any character, and after, if he is ready by card effect, etc., he can be exhausted to attack by hand upon the bow, or to do hide test, etc.

I think there are no rules about it, because it is evident. The only rule is "Characters committed to a quest are considered committed to that quest through the end of the quest phase."

...so you can commit him to the quest and after you can do all you want and can.

Yeah, whether committed to the quest or not, there is nothing that prevents you from exhausting your character for other purposes, if the character is ready.

One more question regarding quest/hide/action timing (because I ran into Fast Hitch / Hide test ambiguity):

In general, people assume that you can commit character to a Quest, and then ready him with Fast Hitch to later commit him to the Hide test. But in rulebooks (Core p. 30, Black rider p.7) is seems player cannot activate Action.

What is the correct interpretation?

There is an action window between declaring which characters are committed to a quest and revealing the encounter cards. You can use this window to ready a character.

Perhaps what you are referring to is that fact that once you start revealing encounter cards during the quest phase there is no action window until after all cards are revealed.