The reasons against delaying mikveh also apply to deliberately delaying a
hefsek taharah. In both cases, a woman is not going to the mikveh at earliest opportunity.
In cases such as you mention, where immersing on the scheduled mikveh night would be very difficult, a couple can agree to
delay immersion.
There is actually no halachic preference for delaying the
hefsek taharah, as opposed to delaying immersion. Therefore, we generally recommend performing the
hefsek taharah and counting the seven clean days as soon as you are able to do so. That way, in the event of a change in circumstances (e.g., a Shabbat away is canceled), you can immerse on time.