Let's define the topic a little better. We may talk about sacs as "logical compartments" in the cell or we can talk about introducing "physical sacs" and their properties into DB universe.
The sacs as logical compartments are already there. For example, memory locations are "sacs" - they are separate from each other and store certain things. The Dna is in a compartment of its own - nothing can touch it at the moment. In the proposed metabolism system we'll have tons of logical compartments. The thing about these compartments is that they can be introduced at will, without any general rules. So there is no need for a concept of a "sac" as there was no need until now.
The sacs as physical consequences of the membrane properties is a whole different matter. Here we should create a more or less general system to describe the properties of bot's borders and/or of bot's internal environments. These sacs would be physically present (visible?) in DBs. I'm not a programmer, but to me it seems out of our reach. After all, current "borders" of bots are simple circles and don't have any physical properties (well, there's been some steps forward in newer versions to introduce those). But what we would be facing are fluid membranes, able to shift around, give bots different shapes (for real), invaginate, fuse and so on... I doubt Nums would be able to code such crazy thing into DB and even if he can, I'm sure it will slow down the program to a crawl...