There;s a problem with the Siena hypothesis, which I, too, had found in many websites and which is fun and interesting and all that. Wherever i found it mentioned, there were no historical references (no footnotes, no research) - none of those i read, anyway. But beyond that, the main problem is that Mascarpone is from Lombardy. Carrying it to Siena in the days of pack mules and no refrigeration would not have worked. That was the most convincing argument. And nobody has yet found a recipe or reference for it before 1960.
Travel articles are written for fun. They don't cite references, references are boring. And even a great chef can be wrong on history, since it's not his field. But the convincing argument would be an actual quote, with the reference material, to some historical document written in in Siena or about Siena in the 1500s or 1600s where tiramisu, or a desert with coffee and mascarpone, appears.
It doesn;t really matter, the question, as Petalsandcoco pointed out, was if it there's a substitute for marsala, and since it's often made without it, and people still love it, the original poster doesn't need to replace it.
Then, you can add whatever you want. There's a famous tiramisu place near here (Pompi), known all over Rome, that makes all kinds of tiramisu', with strawberries, with raspberries, with pistachio, etc, (none of which have liqueur) and which, on Sundays, has a line that goes out on the sidewalk. So if you want to replace it, you can replace the coffee with strawberry juice and put strawberries on top, or in between. You can let your imagination go wild. My only suggestion is keep it simple. The real treat of tiramisu' is the flavor of the mascarpone. Enhance, but don;t cover. I had it once just completely white with no coffee, no chocolate. It was wonderful.