Though the transformer may be protected by the primary side circuit breaker, all the conductors involved still need to be protected. It is not permissible to protect the secondary conductors of this installation with a primary side breaker only, see the last sentence of 240.21(C)(1)
A transformer can be loaded to 100% of its capacity but you;ll have to size the conductors and OCPD at 125% of the secondary full load current in order to take advantage of this. Note 2 to table 450.3(B) says you can have multiple circuit breakers on the secondary side, but they cannot total more than what a single breaker would be.
IMO, the two 100 amp panels described by SJT are not in compliance.
