When subbing, I think you would need to cut the quantity down and perhaps use a secondary sweetener or adjust the acid in other ingerdients.
Most Mai Tai recipes I've seen call for only a quarter ounce of orgeat anyway, which is small to begin with. I've always found that you need to increase it when using homemade orgeat.
Ultimately, you have to ask yourself what the orgeat is doing in the drink. It can sweeten, add an almond flavour or a benzaldehyde flavour, add body through the suspended oils, add opacity. Low-grade commercial orgeat is usually just simple syrup with almond flavouring and maybe colour anyway, so you're no worse off using amaretto, but the flavour is
pretty strong, so you might have to rebalance in some cases. (Or make an "amaretto syrup".) As far as suspended oils, there are other ways to affect texture (using gum syrup, for one). And as for opacity, well, I find it off-putting to begin with, so I don't want to reproduce that.
All of which is to say that, from a functional standpoint, I see no reason you couldn't tweak recipes to substitute amaretto for orgeat. I rather imagine it would work better in some things than in others, though; I'm not sure I would want a brandy-and-amaretto Japanese, for example. But in Tiki drinks? I bet it would work.