[xmlsec] Using different ID type atttributes for sig references efficiently

[melissa collins]

I want to verify signatures that use different types of ID attributes,
e.g. wsu:id, saml:AssertionId etc. in the signature references.
I need to get the API to understand these as ID-type attributes
programatically, i.e. I cannot use a DTD.
I see the xmlAddID() function, however, in order to call this function
I need to find the node(s) with the attribute of non-standard ID-type.
What I really need is a method that you pass the "id name" and "id
namespace" into and that's it. Would prefer not to have to search each
document for nodes with a certain attribute name, and then call
xmlAddID().
Is there a better approach to this that you could recommend ??? 
Thanks,
Melissa