Todd Pattist wrote
But he could legally tow his own banner in a
plane he rents or owns

Not unless he gets a waiver from the FSDO.

91.311 Towing: Other than under §91.309.
No pilot of a civil aircraft may tow anything with that aircraft
(other than
under §91.309) except in accordance with the terms of a certificate of
waiver
issued by the Administrator.

No FSDO I know of will issue a banner tow waiver to a pilot who lacks
a commercial certificate, so the point is moot.

Michael