You can use Job G/L Journals in posting income. You should create a multi-line journal entry where you specify the Job No., Job Task., Job Line Type and other job related fields on the journal line where the Account Type=G/L Account (i.e income account).
However, you would still need to do minor customization on the posting routine. That is, on post of Job G/L Journal where in the journal line the Document Type=Invoice and General Posting Type=Sale, set the Job Ledger Entry.Entry Type = Sale instead of Usage.
Thinking about it loudly, it should work.