1997 quote from EPA white paper on air duct cleaning:
"Chemical biocides are regulated by EPA under Federal pesticide law. A product must be registered by EPA for a specific use before it can be legally used for that purpose. The specific use(s) must appear on the pesticide (e.g., biocide) label, along with other important information. It is a violation of federal law to use a pesticide product in any manner inconsistent with the label directions.
A small number of products are currently registered by EPA specifically for use on the inside of bare sheet metal air ducts. A number of products are also registered for use as sanitizers on hard surfaces, which could include the interior of bare sheet metal ducts. While many such products may be used legally inside of unlined ducts if all label directions are followed, some of the directions on the label may be inappropriate for use in ducts. For example, if the directions indicate "rinse with water", the added moisture could stimulate mold growth.
All of the products discussed above are registered solely for the purpose of sanitizing the smooth surfaces of unlined (bare) sheet metal ducts. No products are currently registered as biocides for use on fiber glass duct board or fiber glass lined ducts, so it is important to determine if sections of your system contain these materials before permitting the application of any biocide."
Most recent white paper on same topic, with NADCA input:
https://nadca.com/sites/default/files/docs/2016/chemical_products_position_paper_webfinal.pdf
I have no further concrete information on what, precisely, will wind up being registered for use in ducts, but I can tell you that several companies have already taken any language related to HVAC systems
off of their labels - most notably, wording regarding the use of antimicrobials or deodorizers on bare sheet metal or plenums in HVAC/R systems. More will be following soon.