I must say that right now there isn’t a better solution (that I’m aware of) other than doing what you’re doing now, but I think that the linked feature request would probably be a big improvement for what you’re looking to do.
To prevent the selling of this product, Prevent selling of a product (a racking-only product) is what you’d need. Until then it would have to be named something obvious to prevent someone from doing it, such as “Barrel ageing stout (do not sell)”. It’s not perfect, but should work in practice pretty well and the product can always be renamed back to something more sensible when the blocking feature is done.
Finally, yes there are plans for a barrel ageing section in Breww, but it’s not really made it onto the community yet… well until your thread now 
I think that if we introduced both Empty containers into a vessel to allow blending two different packaged products to make a new beer and Prevent selling of a product (a racking-only product) this would get you most of what you’re asking for, and we could certainly add an extra feature for logging notes/readings and age tracking. If you agree, can you give those other two features a vote? And we can convert this thread into the age tracking and logging feature?