Ted,
No, it's not possible to parameterize the selection of which input data set to use within a query -- except within a code node, as you mention.
Now, we could get creative with a potential workaround. Suppose you defined a code node like this:
data work.current / view=work.current;
set sashelp.&dataset;
run;
And associated it with the &dataset parameter in the project, which would show a list of valid datasets you wanted to expose.
Then you defined a query that built off of WORK.CURRENT, a view that points to whatever data set was selected in the previous step -- so you have a flow similar to the attached file.
The limitation is that the columns you select in the query would have to exist in any of the potential data sets you used as input. That is, if you assigned CURRENT.NAME to be in the result set, "NAME" would have to be present in any of the input data sets that you surfaced in the prompt.
Chris