How do I code a GL account so that it is subject to a 1099 (payroll GL accounts)?

Not sure if I understood question but QuickBooks online has the option when you are setting up a vendor to select a check box located in the "Edit" box of your vendor page.    Scroll down to "additional info" and you will see a checkbox to select for "Track Payments for 1099".  

To exclude non 1099 portions of a payment, example, an employee/contractor/vendor will not be 1099 for reimbursement, then you need to make sure that the expense has a split on it with a chart of accounts classification that can be excluded when you are filing your 1099 at end of year. 

Example :

Check Payment of $1200 to John Smith

Expense split with $1000 subcontractor pay, $200 with reimbursement classification

When filing end of year 1099 you may exclude all reimbursement classification