If I already control a unit at Trifarian War Camp and play a Fiora Victorious directly to it, she instantly becomes Mighty. Can I then use my Fiora legend to Channel one Rune exhausted?

No, you cannot use your Fiora Legend ability in this situation.

According to the Riftbound rules and established FAQs regarding "becoming Mighty":

1. Trifarian War Camp has a passive ability that grants +1 Might to units at that location.
2. When you play Fiora, Victorious (which has 4 Might) directly onto the Trifarian War Camp, the +1 Might bonus is applied immediately as she enters the battlefield.
3. Because the bonus is applied instantly upon entry, there is no moment where Fiora exists on the battlefield with 4 Might before becoming 5 Might. She is 5 Might the instant she arrives.

As stated in RiftJudge FAQ #5835 and #7974:

• A unit "becomes Mighty" only when its Might changes from less than 5 to 5 or greater while it is already a game object in play.
• Because Fiora is already 5 Might the moment she enters the battlefield, she does not "become Mighty" while on the battlefield. Therefore, the trigger condition for your Fiora Legend ability is not met.

This is distinct from moving a unit that is already on the battlefield (e.g., from your base) to the Trifarian War Camp, which would trigger the ability because the unit was already in play when the Might increase occurred (see RiftJudge FAQ #8556).