I would argue that this feature request is too narrow in its scope. Ideally, manager reservation and booking should respect the same restrictions that apply to an individual booking themselves onto a seminar date. That means (and some of these might already be the case):

• if there is a time period when bookings are open, that time period applies to the ability of managers to allocate spaces to staff.
• if cancelling off of a session is limited to a certain time period, that time period applies to the ability of managers to cancel allocations.
• if a session has a limited number of spaces and waitlisting is turned off, managers shouldn't be able to book staff on over that limit or put staff onto a waiting list that is disabled.
• ideally (and I know this may be a bit out of scope), the manager allocation functionality should respect the Direct Seminar Enrolment plugin audience restrictions, so only staff which would qualify to book themselves on (by virtue of being in the audience) should be allocatable by the manager.

Am sure I've missed one or two, but fundamentally if there is a restriction in place on a seminar, it doesn't make sense for there to be a way for a manager to "bypass" them.