Resolving custom ID attributes

Is there a way to search a document to find custom ID attributes. e.g. 
if a consumer service wants to verify signatures from a producer 
service where the sig is based on a non "standard" ID attribute ("id" 
etc.), say "customID", can the consumer override any Xerces classes to 
look for know ID attributes to verify the signature?
Dom3 has setIdAttributeNS but you need dom3 for that!

thanks,
Alistair