I think your solution is probably the only way to deal with your unusual situation within Sage.  Remember that Sage is designed to be an all-round bookkeeping system - doing an OK job of accounts payable, accounts receivable, and payroll, but without the features of a high-end custom built system for any of these functions.  There can be difficulties dealing with unusual situations.

If you have a number of unusual situations in your payroll, you might want to look at a separate payroll program, such as Paydirt.  Paydirt is very, very customizable, and creates journal entries that can be imported into Sage.  However, you'd then be paying for both Paydirt and Sage.... it's always a question of whether the extra convenience of a payroll-only program for payroll is worth the extra cost.

I've never worked with Piece Rate clients but I would be setting up more than one Income field for the different types of pay possible.

One can be set to Piece Rate and you can set up a specific dollar amount per piece.  Regular Hours or other income field can be used for the Janitorial hours.

Please be aware though that because of the Hourly concept of EI Insurable Hours, if an employee is not paid by the hour, you cannot record hours in the program.  Please see this page regarding how to calculate EI hours for piece work.  I am not an expert, I just Googled it.  So you may want to call the CRA about it.