@ebbe I think this would be a very useful and easy to implement feature request, and I would certainly use or consider using such an option.
My suggestion, is adding a territory option like
<attachment name="territoryAttachment" attachTo="1" javaClass="games.strategy.triplea.attachments.TerritoryAttachment" type="territory">
<option name="isUnoccupiable" value="true"/>
</attachment>
The "isUnoccupiable" option has the same effects as the "isImpassable" option but allows moving through (yet not retreating into) the territory.
I hope such a feature will be added.