There is nothing prohibited about putting "pull the plug", or living will language into a health care mandate (power of attorney), but you will want to be extra careful. Many HCPOAs contain living will language, but it is very common for this language to be outdated. For example, the latest versions of the Louisiana living will ask the person signing to not only sign saying that he does not want to be kept alive artificially, but the person must also make a decision about whether he wants nutrition and/or hydration to continue after the respirator is turned off. If the language in the HCPOA does not contain this language, it will not be accepted by the hospital or doctor in all probability.
Also, doctors and hospitals know that they can depend on the the living will form as it is exactly worded to minimize their risk of liability when life support is terminated. If the language is included as part of a lengthy HCPOA, the question becomes whether there may be some other language in that document which makes their immunity less clear. See LRS 40:1299.58.8. Suppose that you have included the living will in the HCPOA, but the form for the living will is changed by the legislature. You will be forced to change the HCPOA.
What if the client is now mentally incapacitated? They would not be able to sign a new document, and neither of these valuable tools may be accepted by the health care provider. Ouch. The bottom line is that it's a better idea to keep the two documents separate. See LRS title 40:1299.58.1