There is a bit of an incorrect premise inherent in this question because of the use of canned coconut products. It has to be realized, but often is not, that canned coconut is itself a substitute ingredient – sort of like using canned chicken stock instead of real homemade stock, or evaporated milk instead of fresh milk (but probably worse). Canned coconut milk is not something that anybody who is aiming for quality and has any other choice would ever choose to use. In Malaysia or places where coconut milk is available from a local “santan” seller – a local neighborhood guy who freshly grates and squeezes coconuts – you wouldn’t use canned coconut milk unless you are lazy, indifferent to quality, or really short of time. When using fresh, the shredded coconut is steeped in water and what is squeezed out is coconut cream (the first undiluted press) or coconut milk (the further presses further diluted). The stuff in the cans is not the same product, but just a processed coconut byproduct that has been heated and otherwise mangled in myriad ways. This is why it is impossible to get good Malay, Peranakan, or Thai food outside of their homelands – this critical product is simply unavailable in most parts of the world. Spices can be transported, but nobody has figured out a good way to transport coconut milk without pasteurizing, stabilizing, dehydrating, preserving, or otherwise processing it so that it loses its most important properties. While it is perfectly possible to transport fresh coconuts, I’m unaware of any restaurant in the United States or Europe that goes through the trouble.
So to answer the original poster’s question, can you use the liquid at the bottom of the can of coconut cream, the answer is that it would be a sub-optimal practice with an already sub-optimal product. That thinner liquid is what has settled out of the cream after traveling and sitting on a shelf (figure at best a 6-12 month process) and is technically thin coconut milk, but contains settled out components of your whole coconut cream, which the cream will lack if not reincorporated. You can certainly use it this way and save a can of coconut milk, but I wouldn’t recommend it if you have a can of coconut milk handy.
By the way, I consider the tetra-pak coconut cream/milk to be slightly better than the canned. If you can find it where you live, I would recommend substituting it. If you can’t, I recommend looking for a product with minimal added gums and stabilizers.
