This 2017 AOPA article more or less suggests the answer is "yes". However it would be nice to see some explicit reassurance from the FAA on this regard, since the language of the regulation is really quite unclear.
"The regulation 14 CFR 91.225(e) allows aircraft not certificated with an electrical system, including balloons and gliders, not equipped with ADS-B Out to operate within 30 nautical miles of a Class B primary airport—basically, within its Mode C veil—while remaining outside of any Class B or Class C airspace. These aircraft can operate as high as 17,999 feet msl except above Class B or Class C airspace; they also can operate beneath Class B and Class C airspace. "
https://www.aopa.org/news-and-media/...rical-aircraft
Steve