How do I figure baker’s percentage when using mashed pumpkin / potato etc as wet ingredients? Does it count as liquid? Or as part liquid? Or as part liquid, part flour? If part - how much?
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I don’t know, but if you don’t get an answer, I suggest comparing the recipes for pumpkin pie and custard pie or quiche. You should be able to judge by that.
Are you looking to modify an existing recipe by adding mashed veg that isn’t part of the original recipe? I’m a little confused.
My sincere suggestion is trial and error. I don’t even think “mashed vegetables” works as a single category - a potato isn’t a pumpkin.
Or, as @Thimes implied, just find a recipe - it’s certainly been done before.
If you’re going to reinvent the wheel rather than find a recipe, then really reinvent it, don’t be burdened with other people’s guesses.
It shouldn’t be too hard to look up some average water-percentage and fat-percentage ranges for different vegetables - that would partly take care of the liquids and fats question. But the part of each vegetable that isn’t water or fat may act differently depending on what it’s made of. (It’s likely that starchy vegetables will act more like flour than other things will, for example.) But maybe the “kicker” is that nobody’s mash is infinitely fine or perfectly even, and the bigger-than-zero pieces may hold back unpredictable amounts of their water and/or fat, rather than contributing them to the mix.
Sometimes, trial and error is easier. 
