Scott, your description is correct.  The coil is a transformer and works by
inducing a field from primary to secondary.  With direct current flowing
through the primary, there is no collapsing or rising field to induce a
corresponding change into the secondary, therefore no secondary current
flow.  If there is DC flowing through the primary, though, it will overheat
the primary windings due to simple resistive heating of the coil windings.
It is essentially a short circuit to direct durrent, and the current flow is
quite heavy under those circumstances.  In normal operation, where the DC is
pulsed by either the points or an electronic controller, there is a fairly
high opposition to that current flow in the primary, due to the inductive
reactance of the windings, which tends to limit the current flow to a
reasonable level.  Therefore, in normal operation, the coil should not
overheat.  Steve Sandlin