Advanced Search: Search for Aspect properties regardless of content-type

We are looking for a way to customize the 'Advanced Search' so that it will be able to find specified aspect properties on all content types. The evaluator condition cannot be set to a pre-defined content type but has to rely just on an aspect. This is currently not possible.
Background: Our Alfresco certified SAP integration adds some aspects to each document that is connected to SAP - regardless of it's content-type. We now want provide a simplified 'Advanced Search' form to the customers, where they are able to search for all relevant properties of our SAP aspects.
The only way to define a new 'Advanced Search' form in the share-config-custom.xml is to specify a form tag with the name of an existing content type like:
<config evaluator="string-compare" condition="AdvancedSearch">
<advanced-search>
<forms>
<form label="myType Search" description="Search for all documents of myType">myPrefix:myType</form>
<form label="Content Search" description="Search for cm:content documents">cm:content</form>
</forms>
</advanced-search>
</config>

And then, within the type evaluator definition, the fields:
<config evaluator="model-type" condition="myPrefix:myType">
<forms>
<form id="search">
<field-visibility>
<show id="myPrefix:mySAPId" />
<show id="myPrefix:mySAPSystem" />
<show id="myPrefix:mySAPCreator" />
</field-visibility>
</form>
</forms>
</config>

But instead having the types "myPrefix:myType" (or "cm:content") we want to specify either an aspect directly or a form that can be configured to add any properties - without referring to a content-type. At the end, by clicking the "Search" button the search has to consider that all content-types needs to be included.
For example:
<config evaluator="string-compare" condition="AdvancedSearch">
<advanced-search>
<forms>
<form label="myAspect Search" description="Search for all documents of myAspect">myPrefix:myAspect</form>
</forms>
</advanced-search>
</config>
<config evaluator="model-???" condition="myPrefix:myAspect">
<forms>
<form id="search">
<field-visibility>
<show id="myPrefix:mySAPId" />
<show id="myPrefix:mySAPSystem" />
<show id="myPrefix:mySAPCreator" />
</field-visibility>
</form>
</forms>
</config>