I would take a different position on answering the question, and look at the water-hydrazine binary phase diagram. Looking at J. A. McMillan and S.C. Los, J. Chemical Physics 42(1) 160-161 (1965) one finds the experimentally determined phase diagram. With enough water in the mix, one proceeds smoothly from the pure hydrazine melting point down to a slight eutectic near the monohydride, but only 3K lower in temperature.
Further, the same authors have another paper, J. Chem Phys 42(3) 829-834 (1965) that discusses the nonequilibrium phase transformations. The first sentence in the abstract says
Although neither hydrazine nor water supercool to a large extent, their mixtures do. Aqueous solutions of
hydrazine may be strongly supercooled and stabilized as rigid glasses.
In the introductory paragraph, one also finds
Since the phase diagram of this system
is known, it was tempting to look for a correlation
between the irreversible processes of crystallization and
the equilibrium phase transformations. The hydrazine-water
system is particularly useful for this purpose,
since crystallization can indeed be induced in the entire
range of concentration. Samples of different hydrazine
content were then supercooled until glass stabilization
occurred and then warmed up at different rates. The
processes of glass transformation and crystallization
were studied, and values of the free-energy barriers
could be obtained for the latter process.
The rest of this second paper is devoted to measuring and describing thermodynamically the crystallization curves at different water-hydrazine compostions.
So, yes, you can definitely get a glass, if cooled quickly enough, but it just might decide to phase separate on you if it warms up a bit.
For the case of (only) hydrazine-1,1-dimethylhydrazine, the phase diagram is covered in J. A. McMillan, Journal of Chemical and Engineering Data 12(1) 39-40 (1967). This shows a very slow decrease in the liquidus temperature as 1,1-dimethylhydrazine is added, with the eutectic occurring at 94% mole percent 1,1-dimethylhydrazine at 214K, only 61K below the pure hydrazine melting point. There are no compounds. Further, the article states
Unlike the systems of the components with
water, the mixture hydrazine-1,1-dimethylhydrazine
does not supercool
which indicates that phase separation and crystallization of the two components will readily occur. On the other hand, the samples were sealed in glass under nitrogen, and if kept in the dark were stable over a 6 month period.