At the moment you can provide a serverId and select credentials which are used in the settings.xml within the <servers><server> tag.
It would be nice, to have the chance to not just use credentials for server tags, but also more generic for other tags within the settings.xml file. This could be handled like this:
(textbox) Section path: /settings/profiles/profile/
(textbox) Id tag: id
(textbox) username tag: properties/jboss-as.username
(textbox) password tag: properties/jboss-as.password
(Existing textbox) id: aSampleId
(Existing drop down) Credentials