Vegan Alternative to Monte au Beurre?
Yes-ish. Managed to make something similar with margarine, almond milk, flour and nutritional yeast flakes (was going for a cheese-style sauce for mac and vegan-cheese).

Don't ask me for the recipe, I don't have exact ratio but here's my order of operations: boil almond milk, add mustard, flour, salt and any other seasonings you want, then once a boil has reached start adding margarine until it wont take it anymore, then add flour to get a roux consistency, and from here on out just keep whisking continuously then add more almond milk cold and nutritional yeast and whisk continuously until it seems to be coming back up to a 2nd boil. That's it: let cool a it and my vegan cheese sauce is ready.

To make it a vegan monte beurre, you'd want to skip the nutritional yeast (added for cheesiness flavour) and the mustard (added primarily for colour).