Out of curiosity, I looked into it again a bit more...
That flat-eight Wikipedia article in particular has expanded with more examples. The aero engines (Lycoming IO-720 and Jabiru 5100) are boxers with a firing order that indeed implies a third type of crankshaft (as opposed to the two tried by Porsche). Because they're still in use / production, the datasheets / manuals are freely available.
I'd somewhat ignorantly say the Jabiru is a more recent "copy" of the stalwart Lycoming; firing order for both is 1-5-8-3-2-6-7-4.
Given the aero usage, I would imagine the torsion problems associated with the "twin boxer 4" setup that Porsche tried is less of an issue with this "third" configuration. Either that or the low rpm helps. But then the aero types are not small engines, which compounds the source of the torsion: the pistons' inertia. Propellers are not known to tolerate torsional vibration; the Lycoming at least has "dynamic" pendulum / bob-weights on its crank (tuned mass dampers), which would also eventually make an appearance in BRM's H16...
What I find interesting is the timing of all of this. The Lycoming first flew in 1961. The Type 753 in Porsche's 787 / 804 project was started in '61. BRM's ill-fated H16 was '65 into '66, dead by the end of '67, in part thanks to persistent crank failures; Coventry Climax's flat-16 (FWMW) was shelved before the end of '65 after less than a year of development, due to catastrophic crank torsion issues.
Porsche's high-specific-performance, 3-litre version of their boxer-eight (simply Type 908) was '68, and it suffered crankshaft related issues in both configurations; the flat-sixteen was '68 / '69 also. Even though the smaller displacement boxer-eight engines were reliable enough, they always seemed to make less power than anticipated, as though power were being "sapped" somewhere - torsional vibration is known to do this.
This seems to show how Porsche, at the time, twice failed to learn the lessons / follow the example set by others. Or at least it shows an ignorance of the torsion issues that the aviation industry had no choice but to pay special attention to in the '30s / '40s. Bearing in mind there's no straight-forward analytical solution to the piston inertia problem, it can only really be "solved" numerically and by trial and error - which meant lots of tedious, iterative hand calculations for anyone without access to a "computer".
Maybe Porsche would only entertain an eight cylinder boxer again as a sort of last hurrah or homage or race only project etc. They would have to think very carefully about why it didn't work previously and fully explore the possibility space, analysing the situations with the engines from other manufacturers as well. I have no doubt they can make it work armed with today's computer-aided analysis / design approach.
I look forward to it immensely