The Spectrum Analyzer instrument currently provides spectra starting from about 0.5 Hz. Vibration isolation of delicate instruments (like gravitational wave detectors, quantum optics experiments on optical benches, electron beam lithography, etc) deals with signals down to 0.01 Hz and up to 100 Hz. By increasing the duration of the signal to be Fourier analyzed, the lowest available frequency bin decreases accordingly. By reducing the sampling frequency, or decimating the signal (after low pass), this can be achieved without increasing the memory size. An intermediate extension 0.1 Hz - 1 kHz would come easily in the same way.
Giving access to these parameters as one has in offline data analysis would allow to configure the analyzer in a flexible way, allowing online use also in these conditions.