Is there a way to get at the tax model to do some experiments involving different hypothetical tax situations?
Yes, there is. While RSP is open, go to the Excel Format menu and click on Sheet, Unhide, and Tax Tables. Then click on Tools, Protection, Unprotect Sheet. Then alter the first schedule of thresholds and rates, appearing in rows 5 to 9 and columns 7 and 8. Now those rates will prevail if you select the Married Joint button on the input sheet. Be sure to click File->Save As to save your workbook under another name so you don’t loose the original tax tables.