TLDR;
Yes it does indeed "mase"! Note this answer will cover only the masing of comets.
Long answer;
According to this paper on Maser emissions from comets:
In contrast with interstellar OH masers, the cometary OH maser has a well know excitation mechanism. It is a weak maser operating in the linear regime. Its quantitative modelling permits a reliable estimation of OH column density in a first step, the production rate of water from the cometary nucleus in a second step.
Since 1973, the 18-cm OH lines have been observed in more than 65 comets.
Now for the important part which deduces the OH maser:
Studies with realistic modelling of the OH radio lines in comets started with the discovery by Biraud et al. (1974) and Turner (1974) of the 1667 & 1665 MHz transitions arising from the ground state $\Lambda$-doublet in comet Kohoutek (C/1973 E1). Mies (1974) and Biraud et al. (1974) invoked the solar UV pumping of the $A^2\Sigma^+$ excited state from the $X^2\Pi_{3/2}$, followed by subsequent cascades, as the main mechanism to populate the OH $\Lambda$-doublet energy levels. They succeeded to explain the observed line reversals (absorption to emission or vice versa) by the Swings effect, namely the population distribution dependence on the Doppler shifted solar UV spectrum, due to the heliocentric velocity variations.
Following Schloerb & Gerard (1985), the flux $F$ of a OH 18-cm line is:
$$F=\frac{A_{ul}kT_{BG}}{4\pi\Delta^2} \frac{2F_u+1}{8} \iint B(x,y) \int i(s)n_{OH}(x,y,s) \text{ } ds \text{ } dx \text{ } dy$$
To be clear, fluorescence occurs when there is emission of light as the substance had absorbed light or other electromagnetic radiation, while specifically on comets, masing occurs when ultraviolet light from the Sun breaks down some water molecules to form hydroxides that can mase. In 1997, 1667-MHz maser emission characteristic of hydroxide was observed from comet Hale-Bopp.
So basically for comets, masing has to do with hydroxides formed from water molecules.
Occultation event observed with the VLA on 12 February 1997: 1667 MHz (dashed line) and 1665 MHz (full line) OH line variations are observed when comet Hale-Bopp (C/1995 O1) was passing in front of the background source J2013+220. The OH main lines areas are shown versus the distance measure, the projected distance between the background source and the point of closest approach $6.1\times10^5$km (Butler et al. 1998)
The full pdf file website can be found here
You may be asking now, how does the 18-cm wavelength has anything to do with OH maser of comets? Well according to this paper by EPSC - Observations of OH maser comets in 1.6GHz frequency band:
The 18 cm line is the result of an excitation from resonance fluorescence, whereby molecules absorb solar radiation and then reradiate the energy. The OH molecule absorbs the UV solar photons and cascades back to the ground state Lambda doublet, where the relative populations of the upper and lower levels strongly depend upon the heliocentric radial velocity (the “Swings effect”).
Definitions
NOTE: Resonance flourescence is different than normal fluorescence, but related:
the process in which a two-level atom system interacts with the quantum electromagnetic field if the field is driven at a frequency near to the natural frequency of the atom.
To clarify:
An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum.
The "stimulated spectral line emission" definition from Wikipedia:
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.
Now, the 18-cm wavelength used actually stimulates this emission and this paper on A Masing Hale-Bopp explains the rest:
Sublimated $H_2O$ becomes dissociated by solar radiation to create OH radicals. The OH is then excited by solar UV emission lines to decay by fluorescence to upper and lower levels of a rotational ground state.
So in a sense you are correct to say a sort of "fluorescence" occurs, but it does "mase".
Proof
Evidence that a comet mases:
Here as the velocity nears 0, we see a sudden spike in the radio flux, which is in the equation I provided earlier, due to a rise in radiation and decay from the OH on the comet.
A seriously long explanation of the graph can be found on page 3 here.
Additional information from Wikipedia - Astronomical maser#Comets
