(This supersedes the answer given in iQuestion 12613)

Edited 05Apr2014 to reflect behaviour in DraftSight V1R5.1 (Win 7 x64)

First:The "Update view when coordinate system changes" option at TOOLS > OPTIONS... > DRAWING SETTINGS > COORDINATE SYSTEM > OPTIONS appears to control the behaviour of drawing elements within active viewports when the coordinate system is changed.When checked in each viewport, the contents of all entities in the active viewport rotate automatically to mirror the view orientation associated with that viewport.When unchecked for each viewport,  the contents of the viewports do not rotate until the user enters PLANVIEW > A on the command line.

It is recommended that this setting is CHECKED in model space and all viewports if you plan to rotate the contents of any viewport in the drawing set if the previously described behaviour is desired.This can be set up in a template drawing when it is needed on a regular basis.

In a drawing set up as suggested above proceed as follows:Set up the sheet space (paperspace) viewport while in the WCS (the default World Coordinate System) and continue as below:

Create a new CCS (Custom Coordinate System) using the "Entity" option to set the X-axis parallel with an edge of an entity that is to be horizontal in the final view.  (Note that the end of the entity on which selection is made is treated as the starting end and orientation occurs relative to that end.)

If preferred a similar procedure may be followed using other CCS options instead.

If the “Update view …” option is unchecked, enter PLANVIEW on the command line*. (Nothing happens yet!)Enter A on the command line to select the "Active CCS" option and the view will rotate so that the X-axis of the new CCS becomes horizontally positioned.

Rescale the viewport as necessary and lock it.

Note:  Do all the above before scaling and locking the viewport as the CS change may over-ride and disrupt other viewport settings that are otherwise secure when the viewport is locked.

* The alternative to typing PLANVIEW is to reset the "Update view when coordinate system changes" option for the viewport before creating the CCS.  In this case the view will automatically rotate to reflect the new CCS.