Excellent idea and potential for multiple products. Rather than incorporating a thermometer, might be even better to have a "port" or two that is sealed, but could be used for a thermometer, probe or plug to measure temps or run a hear source for the cold areas.
I second the Keg sized bag to be used for mashing. Some people ferment in a Sanke as well and I'll be using a 15 gallon poly barrel for 10 gallon batches, this could definitely be useful.
Good luck with the product!