I worked this out and made a Java program out of it.
You can use the output for the command line for Multivalent-Impose command, -page option

Command Syntax is : java Page <pages to print> <pages to pad>

So, like for my example 1 of 10 pages, I'd enter "java Page 10 1" because I want my first page as a left-hand page. The output is:

For 10 pages on 3 sheets
x,x,1,10,9,2,3,8,7,4,5,6

The two "x"'s tell me that if I use the document as is, I will have two blank pages in the final booklet for a total of 12 pages. If I say "java Page 12 0", I get:

For 12 pages on 3 sheets
12,1,2,11,10,3,4,9,8,5,6,7

By comparing the sequence of numbers, I can determine I would need to add a page to the beginning and end of my 10 pages to get 12 pages in the order I want.

Here's the source. Holler if you have questions.

import java.io.* ;
import java.util.* ;
import java.lang.Integer ;

public class Page
{
	public static void main ( String args[ ] )
	{
		int pages = Integer.parseInt ( args[0] ) ;
		int pad = Integer.parseInt ( args[1] ) ;
		
		int n = ( pages + pad ) / 4 ;
		
		if ( ( pages + pad ) % 4 != 0 ) n++ ;

		System.out.println ( "For " + pages + " on " + n + " sheets" ) ; 

		Integer num ;
		int foo ;
		ArrayList pageOrder = new ArrayList ( pages + pad ) ;

		for ( int m = 0 ; m < n ; m++ )
		{	pageOrder.add ( new Integer ( ( 4 * n ) - ( 2 * m ) ) ) ;
			pageOrder.add ( new Integer ( ( 2 * m ) + 1 ) ) ;
			pageOrder.add ( new Integer ( ( 2 * m ) + 2 ) ) ;
			pageOrder.add ( new Integer ( ( 4 * n ) - ( 2 * m ) - 1 ) ) ;
		}

		for ( ListIterator it = pageOrder.listIterator() ; it.hasNext() ; )
		{	num = (Integer)it.next() ;
			foo = num.intValue() - pad ;

			if ( foo <= 0 || foo > pages ) 
			{	System.out.print ( "x" ) ;
			}
			else
			{	System.out.print ( foo ) ;
			}

			if ( it.hasNext() )
			{	System.out.print ( "," ) ;
			}
		}
		System.out.println () ;
	}
}

[update] fixed a typo