Consider:
1) John is a Fighter with Parry. His friend gets hit with an Eviscerate. His Parry is applicable because his friend is within weapon length; can he Bane it?
2) If Mettle must consume 20 Body to be applicable, then sure, you should have to pay 20 Body. But if the only thing that needs to be applicable is the effects it stops, then no, the body payment is irrelevant.
I feel like #2 will mean that #1 is legal.
So this is my interpretation of Retribution: Mettle, with RAW. I leave intent aside for the moment.
Premesis
P1: "The character may not use this skill if they do not currently have at least 20 Body Points, [...]"
P2: "[...] nor may they use othe skills, ritual or abilities to reduce or negate the damage."
P3: "When struck by [...] the character may call "Mettle" as a defense and expend 20 Body Points. This immediately negates the effect."
P4: "This ritual allows the user to, once per Logistics period, use a Return instead of a Guard. The Guard used to trigger the Retribution ritual is expended along with the Retribution, and may come from any applicable skill, Ritual, or other effect. When this Ritual is used, the verbal will be “Bane”."
Theses
T1: Per P1: If you don't have 20 Body to expend, Mettle is not an applicable defense.
T2: Per P2: The expenditure of the 20 Body Points cannot be reduced by anything, including the use of the Retribution ritual.
T3: Per P3: Expending the 20 Body Points is
required for Mettle to be an applicable defense. It is the expending of 20 Body Points alongside the call of the defense itself that negates the effect. (Guard).
T4: Per P3: Calling "Mettle" is required for Mettle to be an applicable defense.
T5: Per P4: When Retribution is used the verbal is "Bane" but it does not state to use this instead of the verbal of the Guard.
Conclusions
C1: Therefore, when using Mettle to trigger a Retribution, you must have the 20 Body available, and expend the 20 Body Points, or else the Mettle was not an applicable defense.
C2: By RAW the proper call for this would be "Mettle, Bane". (I realize this may not be RAI, but it's the conclusion I have to come to from RAW)
Extrapolation
E1: By RAW the proper call in the general would be "(Guard Defense), Bane".
E2: Yes, you could Parry then Bane an Eviscerate. I base this on the idea that a logical and legal chain would be Intercept -> Weapon Shield -> Retribution and what is a Parry but Intercept + Guard in one package?
E3: In the Intercept case the call would be "Intercept, Weapon Shield, Bane".
@__@
I just spent an hour chunking through the logic here. What am I dong with my life?