The only bands to ever make finals at SA after missing at a regional are RR (Austin 2019), VR (Austin 2014), Plano East (Denton 2012), Aledo (Arlington 2010), Ronald Reagan (Arlington 2009), Leander (Arlington 2008 & 2009), Hebron (Arlington 2005 & 2007), Brazoswood (Pearland 2007), Coppell (Arlington 2005), and Lake Highlands (Arlington 2004).
Of these, only Aledo (15th -> 14th) and Lake Highlands (17th -> 13th) improved on their placement from the regional to SA.
So basically, it used to be fairly common, but it’s become a lot more difficult in the past 5-10 years or so.
Feel free to let me know if I missed anyone! Knowing me I probably did