if you do have proper extension, than it is simple :)
You could create your servletExtension as part of your deployment processor, where you can lookup service registry by calling deploymentUnit.getServiceRegistry() and lookup service from there.
once you have created your servletextension with the data from your service just pass that servlet extension instance to attachment list.
deploymentUnit.addToAttachmentList(UndertowAttachments.UNDERTOW_SERVLET_EXTENSIONS, your servlet extension);
this way your exact instance will be used, no need for registering it as service in meta-inf...
hope this helps,
tomaz