Idli and dosa are fermented batters ,so its inevitable they had little shelf life .Theoretically freezing will drastically slow down the microbial activity ,but there still the tendency of water separation and disintegration of structures due to starch retrogradation and formation of ice crystals which ruptures the fermentation created gas bubbles.
The microbes viability upon thawing needs to be checked…
You better off with making a dry premix of your idli batter inoculated with microbial spores so you can make any dosa or idli batter when you need it.
You just hydrate the dry mix allow it to ferment and there you go,you got a consistent quality product for your needs.
Regarding the pure culture of idli and dosa lactobacteria, you should consult CFTRI in Mysore Karnataka.if they can help you with it .