You can't get a GL account to require but you can use a script or change settings in other process such as AP invoice data entry to require a job number. 

Yes, I think we've done that script before...

Checkbox in GL Account for "job required", then a Pre-Write line script (in the applicable data entry type) to lookup the checkbox, and SetError when the field is blank and the line's GL account is set as Job Required.

Yes, I think we've done that script before...

Checkbox in GL Account for "job required", then a Pre-Write line script (in the applicable data entry type) to lookup the checkbox, and SetError when the field is blank and the line's GL account is set as Job Required.