Note: I'm looking for explicitly Thomistic resolutions of these case studies.
Alright, from poking around Feser's blog, I get the feel that the litmus test for considering something to be suicide would be whether your goals in undertaking this action would still be fulfilled if you were made impervious to death.
Now let's get down to business. I think this particular case is a little edgy, so here we go.
Imagine you're on a high-tech sci-fi space station, with thousands living on it. It has just been attacked by terrorists, and is in danger of being destroyed if its systems are not brought back online pronto. The technicians are working their butts off, but they'll still need a few extra seconds, otherwise everything goes up.
The space station has been leeched dry of energy. The only way they could get an extra jolt of energy to keep the system stable for just a few more seconds is by chucking something into the mass-energy converter which is for some reason linked to the station's grid.
Unfortunately for you, you alone are right next to it. There is nothing else of sufficient mass-energy around you, and you've only got a few more seconds before disaster strikes. The only way to save the station is to chuck yourself into the converter, in a heroic sacrifice.
Which brings us to the question in the title: is this suicide?
On the one hand, this entails your destruction. It requires you to convert your entire body to mass-energy. You intend to destroy your body, so it seems to fail the Feser-derived litmus test.
But on the two hand, suicide is a sin because it indicates a total and utter despair and self-abandonment. And this can be resolved to be something a little like cutting off your arm to escape a Saw-esque trap, except far more extreme.
So once again, to all Thomists out there: is this suicide?