Reference another query via a join using the IN function, is this possible?

I am fairly certain that you cannot reference another Power BI Query from within a SQL statement using the Advanced feature of the connector. You can reference another query within a different Power Query query (M code) but once you use the SQL code feature it is M code that goes out and executes straight-up SQL code. 

ImkeF or someone else, please correct me if I am off base here.

I'm having difficulties to understand what's requested here.:

But you can include results of your M-queries in SQL-statements: As text.

let
    Source = Sql.Database("localhost", 
			"AdventureWorks2008R2", 
			[Query="select * 
				from [Production].[Product] as [_] 
				where [_].[ProductID] = " & Text.From(ProductID_QueryResult)])
in
    Source

So here for example, I'm referencing a result of my query "ProductID_QueryResult" as a parameter for the ProductID in the SQL-statement. I have to transform it into text, because that's the only way to integrate it in the SQL-code.

So if you want to integrate lists, you have to transform them into text-expressions first.