What you mean by planar projection?
My first idea is to make hotspots size handler that will on each view changed be triggered by event and based on calculated zoom level (make it from 1-10 based on min and max values) and with some new attribute in the each hotspot definition e.g. show_at_zoom_level="4" go through all hotspots and turn off all of them being defined to be hidden when level is to low.
Umalo

You can see what I had in mind below. There are other ways on how to do this but this came to mine mind first.

Link to online example of hotspots with visibility and size related to zoom level: link to online example
Code you can see: link to xml

Of course you have to adapt the code to your pano but to give you idea one one solution for you request.

Don't know what you did with the code so can''t give you more than:
- There is onviewchange event used that is checking current zoom level and make some hotspots invisible.
So it might be that you have to remove this event before loading new pano not to be active on your new scene and not making your hotspots invisibe there too.

Easiest way is to make this event not global but local:
Change
<events onviewchange="checkfovchange();"/>
to
<events name="somename" onviewchange="checkfovchange();"/>

Without keep="true" it will be removed when loading new pano.

Reference for you: http://krpano.com/docu/xml/#namedevents