There is no such thing as “secret rule making”
A known asshole congressman got hold of a draft proposal and is using it as a fund raising /attention seeking tool.
Congress passes statutes and the executive agencies which implement them develop rules to implement the details of how they are implemented consistent with congressional intent. Those rules are drafted internally (the stage where the subject of the bullshit, inaccurate TFB article is currently) if they are considered for implementation they are proposed publicly in the federal register and subject to a public comment period. After taking public input into consideration the proposed rule is either scrapped, modified, or implemented by publishing in the federal register and included in the appropriate chapter of the CFR or Code of Federal Regulations. At that point if the rule is unreasonable, unconstitutional etc it can be challenged in court.
This process applies to all federal agencies / regulations it is not unique to ATF nor is it a “secret.”
PS - the proposed rule applies to only one specific make /model of brace, not braces in general.