This is happening, I have confirmed it. It's been posted for a week, apparently; just not so long on Facebook, which is where I ran across it in a group there.  

The four Missouri "2A sanctuary counties" county commissioners are all attending this event as well.

I'm going to try to be there.

Concealed-carry in the Capitol...

https://mcp.dps.mo.gov/capitol-security.html

"Those with a valid conceal and carry firearms permit must use the South Carriage Tunnel public entrance and be in possession both of their permit and valid photo identification. Conceal and carry permit holders are not allowed to carry concealed firearms on the chamber floor of either the House or Senate, in the House or Senate galleries, or any legislative meeting rooms. This policy is in accordance with Chapter 571 of the Missouri Revised Statutes.

Persons who do not meet the above qualifications may not enter the Capitol Building with a firearm. Persons in legal possession of a firearm but who do not meet the above qualifications will be asked to return the firearm to a secure location in their vehicle prior to entering the Capitol."