I used to see guys jumpseating in on a late flight to do earlys the next day after only a few house sleep then once the 5 day pattern was over they would jump seat home on a late flight after an early duty. These guys looked constantly fatigued.
I believe it does say somewhere in Part A that your not supposed to do this but I would guess that's only to prevent canceled flights disrupting the schedule in case you don't make your duty
A good friend of mine was once told by his base captain that he will be removed from his duty because he flew in on a late from STN to do an early the next day but ended up doing the duty still because the base captain was told it's none of your business how he gets to work. He could have driven all through the night and you would not have known.
Legal or not I can't say. Personal opinion is that it most definitely should not be allowed. You have to be in full uniform and available to assist with monitoring refuelling or walk around a etc so I suppose that makes you active crew. However it does not count as FDP! So there not paying the game right that way I suspect.
My guess is that if it was removed as a privalage and had to be rostered it would cost Ryanair so much time and hassle that the 5/4 roster would have to be scrapped and that would lose them a huge amount of crew that only work in FR because of the jumpseating 5/4 option. Likely planes would be parked up due lack of crew and expansion slow or even retract. You can't have that many airframes without the crew.
A lot of smaller Italian bases struggle for crew as no one really wants to be there. 5/4 you can jump in and out and earn a crust. Lose the option and i would guess most will jump out and apply elsewhere