I am looking for an electronic tool that helps answer the question:
"Given all the  relevant economic variables, does it make sense to
payoff a mortgage early?"  The tool enables 'what-if' analysis and
takes into account such variables as interest rate, projected ROI for
market investments, tax bracket etc.

I don't have an answer for you, but I do have a question.

Your question assumes there might be a reason to NOT pay off a home
early. I'm curious. Why would one NOT want to pay off a home early?

-JtB

You might want to do a Google search under:

intrinsic value of house "gary smith"

Smith wrote a program that calculates the intrinsic value of a house
as an investment. It takes into consideration cash flow, internal cash
flow, tax brackets, interest deductions, rent vs. purchase, and so on.

In the returns on his program you will see a point at which your
return on investment (given all of the above and more) will become
less than a fixed investment in, say bonds or stocks. At that point it
pays to rearrange your investment priorities, pay off the mortgage, or
refinance and invest elsewhere. Also, given the conditions in certain
markets like San Diego, San Francisco, Manhattan, etc., it actually
pays to rent rather than own, which is detailed in his writings and
software.

P.S. I believe that whomever runs the web site www.interest.com has
purchased a license to his program. They have a free online version at
their site.