Interesting take on it, considering that we both looked at it the same way: we both said that what would make it grave (that is, in the context of a
substantial observation (oops, sorry! – ‘substantialis observantia’

) of the obligation), would be the intent: as you mentioned, it would be whether the hypothetical person’s decision was prompted “by utter hatred of ecclesiastical authority”. That’s a bit of hyperbole, of course; but it touches upon the salient issue – did the guy say “I know it’s Good Friday, and the Church says ‘no meat’” or merely “meh, I want a steak”? The OP indicates it’s the former: “he knows it is a mortal sin to succumb to his craving today but he willfully decides to eat one anyway”.
I agree that, perhaps, we’re not seeing the scenario in the same way – yet, our analysis of the salient issues seems to coincide (unless you see “utter hatred of ecclesiastical authority” in the Latin which you are quoting

). The issue, it seems, comes down to precisely what the OP is asking us: is it the ‘eating of the meat’ or the ‘obligation’ that’s in play here? You offer a reasonable hermeneutic: that of mercy. Yet, in quoting the text, you seem to substantiate a different hermeneutic: one of substantial adherence to a precept. These two seem at odds… at least in the hypothetical situation offered by the OP.
Quite. And, a first year theology professor would rightly offer that critique. Yet, we’re not asking what the text says… are we? We’re asking what the hypothetical situation is offering us for consideration. You and I seem to agree – although you seem unwilling to admit that this is the case: if the person, in his decision, is saying, “I want beef”, then that seems somewhat harmless; but, if they are saying, “I do not wish to conform to what the Church obligates me to do”, then there is a particular conclusion that may be reached.
(Unless, of course, you wish to say that, for those who ‘wish to conform’ five weeks out of Lent, but ‘wish to refuse to conform’ on another week, then there’s “no harm no foul.”)
I think I understand what you’re saying; and yet, if I may caricature it in order to probe how well it holds up, it seems that you’re suggesting that, at a Good Friday service, you wouldn’t blink at a priest who tells his flock, “hey… as long as you’ve
substantially held to the observation of Friday abstinence during Lent, you might as well go out to your local steakhouse and enjoy a nice big porterhouse steak tonight!”. Is that consonant with your notion of ‘substantialis observantia’? If not, then how would you distinguish between these hypotheticals?