I am interested in accepting a personal loan from a friend (as a
borrower), but the rate is above my state's usury limit. Can I enter
into an agreement that says something like: I excuse my right to claim
this loan usurious and will not be able to use the usury state limit
as an excuse not to pay? Basically, can I forefeit my right to rely on
state usury law and still have a legally binding contract?

"Restrictions on legal interest rates apply to banks, consumer loan
companies, and other businesses that extend credit. Loan agreements
between private individuals are also governed by state usury laws. For
example, if a person agrees to lend a friend $5,000, the interest rate
cannot exceed the maximum set by the state usury statute. Persons who
charge excess interest and then threaten extortion are known as loan
sharks. They may be prosecuted for usury and, if convicted, fined and
possibly imprisoned."
http://www.answers.com/topic/usury

Your state laws may not include private party loans, but they probably do.  

I'll mention that lending money (especially at high interest rates)
between friends is often a quick way to end the friendship.  Money
often gets in the way of relationships, and if this loan is in fact
usurious then there is a good chance it would put a strain on you 2 at
some time.

It the amount you are seeking is $ 25,000 or less you could apply for
a loan at www.prosper.com have your friend become a lender and make
good/finance the loan. You can then set the interest rates you want
(pretty much). And, you get to use them as the intermediary, making
everything legal.

Good luck.