Applying a border to an image in FrameMaker that doesn't show in RoboHelp?

Unless something has changed in later versions, you can't apply a CC to just a border (or even to an imported object per se). You can only tag text, in the case of an imported object, the anchor. Now, that could be made to work if you are willing to have two versions of the object. One has no border, and has an anchor CC for the RoboHelp workflow. The other has a border, with an anchor CC for other workflows.

What also might work is to apply a named color to the border, which is defined by Color Views to be Invisible when rendering to the RoboHelp workflow.

Generally, Color Views is easier to manage for multiple output workflows (or was before FM10's standard scripting, which might allow you to reduce the rendering mode to a keystroke, no matter how implemented).