GURPS Power-Ups 9: Alternate Attributes does discuss the subject. It doesn't do exactly what
"The Fifth Attribute" does, though. Rather, it offers a shotgun blast of possibilities:
If you make spells depend on an attribute I'll call "Power" rather than on IQ, then you've effectively split IQ in two. The suggested price range for Power and IQ then becomes 11-20 points. Power should never cost
less than the 10 points/level of Magery or Power Investiture, while IQ could continue to cost 20 points/level.
No level of Power
automatically grants spellcasting ability! Each realm of magic (mana-based, sanctity-based, etc.) requires an Unusual Background to "switch it on." That costs whatever the GM believes "I can work magic!" is worth – at least 5 points, possibly a lot more (10, 25, 50 . . .).
Power being an attribute doesn't imply that just anyone can reduce it. The GM is free to permit only those who've switched on spellcasting with an Unusual Background to do that.
For gamers who dislike such fiat solutions, the alternative is to make low Power
bad. The simplest way to do so is to have resistance to spells use the worst of Power and the usual attribute (usually Will or HT) . . . and to make this problem
obvious to spellcasters!
Also, there can be uses of Power other than spellcasting. If these matter to non-casters, then letting everyone lower Power becomes fairer.
One such use is "everybody gets a Power roll to sense curses and magic." That makes low Power a problem both on an economic level (you miss valuable magical loot) and on the level of personal safety (you don't sense curses and magical traps). This roll could be penalized for non-casters without suitable UBs, but letting everybody at least
try makes lowering Power a risk.
Another such use is to have all supernatural powers – and even weirder
non-power abilities, like Danger Sense – use the Power score for all relevant attribute rolls. You can have all the DX, IQ, Per, etc. you want, but all of your supernatural senses, attacks, and so on rely on Power. Lower it and you can never be a capable
chi-user, priest, psi, etc.
Yet another is that skills covering esoteric subjects move from their current controlling attributes to Power. This means spells, obviously . . . but also all cinematic martial-arts skills, and perhaps even stuff like Autohypnosis, Dreaming, Enthrallment skills, Esoteric Medicine, Exorcism, Meditation, Musical Influence, Ritual Magic, and Thaumatology.
As well, Power becomes the base for whatever points are used to energize spells and supernatural abilities – call them Energy Points. EP start equal to Power, just as FP start equal to HT. And no, you
can't spend FP on spells and abilities!
Lowering EP below starting level is best handled by simply stating you can't reduce EP by more than 30%. The GM can say "You can't lower EP at all!", but that isn't necessary. Introducing attacks and spells that damage or drain EP, and that have
very bad effects at negative EP (like HP loss, HP
and FP loss, or even Will loss), can make low EP a serious problem.
And of course the GM is free to say that EP power
all the abilities and skills that Power controls: advantages (like Healing), cinematic martial-arts skills, Enthrallment skills, spells, etc. The GM might even move "cinematic" uses of FP to EP, meaning things like extra effort in combat and
TV Action Violence (p. B417).
With enough hanging off Power, the GM can let it work like any other attribute, because those who lower it won't be able to exploit unusual skills and abilities, and will become vulnerable to the supernatural. This is only recommended in campaigns with lots of such capabilities and dangers . . . but there, lowering Power or Energy Points is really no more or less bad than lowering, say, IQ is for a dedicated warrior.