It would help if you would tell us what form of output you're using the Multi-object buttons for—SWF output, DPS output, EPUB Fixed Layout Output?
However, I think in all these forms of output, button hotspots are always rectangular.
I'm shooting for a simple Interactive PDF... No bells or animations. I'm trying to avoid the animation I know would very easily create the pop-up I'm trying to get from the roll-over button state. The problem is that I want multiple pop-ups to happen in the same area for multiple buttons. Even with a "blocking object" (my term), the button that is highest (foremost) in the layer blocks all below.
Simple-Stupid just isn't very impressive, but we all have boundaries to work within... I suppose.
To clarify, you're talking about creating buttons on the Buttons and Forms panel, right? (Your use of the terminology "multi-object button" made me think you were talking about multistate objects created with the Object States panel.)
Animation isn't possibly anyway with interactive PDF.
It sounds like you probably want Show/Hide buttons. Here's are some tutorials:
@CrzAdler – The activation area of a single button in InDesign is always a rectangle.
A workaround could be using several small buttons (with the same properties) to fill an arbitrary form.
This would be no perfect solution and is tedious doing it without scripting. *
Note 1: Flash (and/or ActionScript 3) is able to produce buttons with arbitrary activation areas.
But I did not test these with Flash app's feature to convert them to HTML5.
* Note 2: An algorithm filling an arbitrary shape with different sized circles (converted to buttons) could be a solution.
See what Jongware was doing here in Illustrator by scripting (no buttons involved, just creating the shapes):
Maybe this AI script is something for you. Filling the shape in Illustrator and copy/paste the circles to InDesign converting them to buttons afterwards.