would mixing Sodium Hypoclorite with NaOH evolve Chlorine Gas?
No chlorine gas will be formed. Hydroxide ions stabilize hypochlorite ions; this is why commercial hypochlorites are nearly always sold as solutions with NaOH as well. If your 'sani-flush' product happens to be a drain opener type of product (for toilets I would assume) it will likely also contain sodium silicates (as abrasives and as a precipitating agent) which will prevent you from using this product as a viable source of hypochlorites.
Check the bottle for an ingredient listing. If one is not found, call up the manufacturer customer service (number on the bottle 'questions or comments?') and ask them if the product contains silicates.
-T
Sani-Flush MSDS (http://www.wesclean.com/FTP/MSDS/319003.pdf)
(http://www.wesclean.com/FTP/MSDS/319003.pdf)