I think I would argue that it is in future tense, in all contexts.
In Matthew 26:26-28, in the NAB, we have:
While they were eating, Jesus took bread, said the blessing, broke it, and giving it to his disciples said, “Take and eat; this is my body.” Then he took a cup, gave thanks, and gave it to them, saying, “Drink from it, all of you, for this is my blood of the covenant, which will be shed on behalf of many for the forgiveness of sins.
You mention the Latin. If I’m not mistaken, the same ending is used for 3rd person singular passive present and future, isn’t it? Therefore, on the face of the text, we would be able to conclude ‘future’.
But, since the Latin comes from the Greek, what’s the Greek text say?
*τοῦτο γάρ ἐστιν τὸ αἷμά μου τῆς διαθήκης τὸ περὶ πολλῶν **ἐκχυννόμενον *εἰς ἄφεσιν ἁμαρτιῶν
Uh-oh. It’s a participle. As it appears in the sentence, ἐκχυννόμενον (“is being poured out”) is in the present tense. But, if we want to understand what a Koine Greek participle means, we have to look at the verb it’s modifying. After all, in Matthew 23, we have the exact same participle, in the exact same grammatical form:
“Woe to you, scribes and Pharisees, … behold, I send to you prophets and wise men and scribes; some of them you will kill and crucify, some of them you will scourge in your synagogues and pursue from town to town, so that there may come upon you all the righteous blood shed upon earth, from the righteous blood of Abel to the blood of Zechariah, the son of Barachiah, whom you murdered between the sanctuary and the altar.
In the Greek: *ὅπως ἔλθῃ ἐφ’ ὑμᾶς πᾶν αἷμα δίκαιον **ἐκχυννόμενον *ἐπὶ τῆς γῆς.
It’s the same participle in the Greek, and it’s in the present tense, but here the meaning is clearly that the blood had already been shed (after all, it’s Abel’s blood, and Zechariah’s blood, etc).
So, what’s up with that?
In the Greek, we have to interpret participles in light of the main verb. In the Institution Narrative, the verb is “is”. (“This is my blood…”) So, inasmuch as it truly is His blood, then the participle follows that tense. However, I think it’s reasonable to recognize the shades of meaning here. Jesus’ blood is literally shed on Calvary, but if we take Him at His word, it’s also present in the Eucharist. (Sacramentally present, mind you, not present in the mode in which our blood is present in our bodies, but truly present nevertheless!)
In English, can we use a present tense form to refer to a future event? Actually, the answer is ‘yes’. Take this example: “the train leaves at 6pm tomorrow.” In this sentence, we clearly have a present tense form, but it’s being used to refer to a future event.
I might make the case that “blood… which is poured out” is being used in the same way. In the English, we have a present tense form, referring to an event which is in the future to the speaker.
So: I’d go with “future” meaning all around. (And, of course, sacramental mystery as well, inasmuch as He’s pointing to the chalice that’s right there in front of them as He says it!)