Hmm, the only trick I have in my book is you can use SAS/ACCESS to PC Files and the Excel Libname statement to write to a new worksheet. For example:
libname myxls "C:\data\states.xls";
Then, any task/program could output to the myxls library *as long as* you picked a name that wasn't already the name of a worksheet. That's a big caveat: you can't have a worksheet with that name already in the workbook. So, you're not really outputting into a worksheet, you're creating a new one.
I'm sure there are other solutions, perhaps along the lines you describe. Hopefully we'll get a few more people weigh in!