FMA didn't have "meta" aspects, but, considering the way he wrote the explanation for equivalent exchange and the gate in his original script for the anime's ending, compared to the manga's explanation, and also considering his own first episodes in Decade, those nebulous concepts fit his style.
IIRC, the first anime series's explanation for equivalent exchange was not Sho Aikawa's idea but director Seiji Mizushima's, suggested at a meeting of all creative talents involved in the show. One of Mizushima's passions is working real-world history into his TV shows.
I think Aikawa's Decade would not be devoid of metafictional content, since Decade was never really devoid of it. The moment a crossover becomes totally self-aware the way Decade's was from its first episode, you're reminding the audience that the fourth wall exists.
Subsequent Aikawa episodes didn't really wallow in the metafictional concept, instead using it as a framework to work within. I suspect that Aikawa might have used metafictional means to explain why Tsukasa met alt-Riders instead of the known canon Riders, though.
Aikawa is generally a writer who likes to explain things and affects a clean, concrete style. I think his Decade plot would have been less insanely ambitious than Yonemura's, a loss in some ways, but it definitely would've been clearer about who Narutaki is, what Kivala really is, etc.
This is all pretty pointless speculation, though. No matter how well you think you know a writer's style, they can surprise you for reasons you never would've imagined. Or they can end up working under creative pressures that substantially change their approach.