Not in particularly simple fashion unfortunately. The lines where a guess is read is here: https://github.com/psi4/psi4/blob/master/psi4/driver/procrouting/proc.py#L1275
The relevant pieces are
Notice that it wants occupied orbitals only. The
reset_occ flag is if you want to set a weird sized starting occupied orbital, the wavefunction will reset itself to a reasonable occupation count (particularly useful for incomplete cholesky guesses of a density).
If you want to play around with guesses there is the Psi4NumPy repository. The linked version is just as fast as the Psi4 one due to the JK builds through Psi4; however, you dont have to deal with all of the user-facing logic that makes many Psi operations simple.