A colleague sent me the following information regarding this question:
Actually the documentation does mention that a time-varying
measurement matrix, H_t, is permitted. All you need to do is provide H_t
that is a ((T+lead)*N_y) times N_z matrix where, T is the number of time
points, lead is the forecast lead, N_y is the dimension of the observation
vector, and N_z is the dimension of the state vector. For
example, in the user’s notation, if the observations are univariate, the
rows of H_t matrix will be [1 M_t], t=1 …(T +lead).
By the way, as a side note, mention to the user the new state
space modeling procedure, SSM, in 9.3 (its doc is at http://support.sas.com/documentation/cdl/en/etsug/63939/HTML/default/viewer.htm#etsug_ssm_sect006.ht...
).