A parent of a student who is registered under a private (600) school has her child taking band at our local public high school. I don't believe her child can participate under the "home ed." statutes.

You are correct.  The FL School Music Association, which oversees band and chorus, has the same eligibility standards as FHSAA.  They used to be one in the same when the Florida High School 'Athletic' Assoc. became the Florida High School 'Activities' Assoc. The guidelines of the Craig Dickinson Act (FL St.1006.16) are the same in both cases, and when the FSMA moved out from under FHSAA, they took along the same standards.   

That said, some counties do allow private school students to take classes at the public schools.  However, the band competition is what would make this situation fall under eligibility standards. 

This is one of those situations where the burden to do the right thing falls on home schoolers.  ALL home educators could lose the opportunity to participate in extra-curricular activities once ONE team/band/chorus loses its standing because one of "us" was ineligible.  The FSMA website states:

2.3  A student is eligible to participate in FSMA events if that individual:

• 2.3a. is not a high school graduate; and
• 2.3b. is a full-time student in the participant school, or has registered as a home-schooled student with the participating school.

Here's the link: http://www.flmusiced.org/FSMA/2003-04rules.html