Except, looking at the RIF regulations, there is no way for RIFs to be accomplished while the government is shutdown -- not unless you are going to bring a number of workers back to perform the RIFs. To quote from the
OPM policy pages, "An agency must use the RIF regulations before separating or demoting an employee because of an organizational reason such as reorganization, including lack of work, shortage of funds, insufficient personnel ceiling, or the exercise of certain reemployment or restoration rights. In fact, virtually all RIF actions are the result of a reorganization (e.g., the agency reorganizes as the result of a shortage of funds, lack of work, restructuring, etc.)."
As such, unless the plans for reorganization, for basically every department in the government, are already in place and were started to be implemented before the shutdown there seems to be little chance that a RIF could be successfully made. This becomes even more true with the short lead time between when Trump made the decision to shut down the government and the government being shut down (and if they had been made, it likely would be reported on now).
The fact remains, RIF is a process that needs to be completed and, without the people at work completing the process, it would appear that legally the required process cannot be completed. Now, I'll admit, there is a great deal of law that covers the RIF regulations and process (5 U.S.C. 3501-3503) and I don't have the time or inclination to read though all of that to find what loopholes and exceptions there are. From what I'm reading, however, it does not appear to be legally possible to do a RIF based solely on this shutdown.