Is a negative Es/No for an SCPC RF signal possible, and what would such a value signify?
Assuming the number is reported in dB, a negative value of $E_s/N_0$ means is that the energy per symbol $E_s$ is less than the noise spectral density $N_0$.

Even then, a negative $E_s/N_0$ seems hard to obtain if the signal's power is larger than the noise power (as measured on the spectrum analyzer). Assuming that the maximum symbol rate is used ($E_s=2B$, where $B$ is the bandwidth), and that the noise power $\sigma^2=N_0B$, then $$\frac{E_s}{N_0} = \frac{P}{2\sigma^2},$$ where $P$ is the signal power.

So, under these assumptions, a negative $E_s/N_0$ (in dB) means that the signal's power is less than twice the noise power.