Some degree of JSP code completion is working its way into RSTALanguageSupport. Currently only code completion/inline documentation for the standard JSP tags are baked in, but coming soon will be support for importing taglibs. The code is actually mostly there; the problem is figuring out how to expose it in the API in a simple, clean way. It’s a little tough since RSTA is just a text editor component; for an editor application with the concept of a project or workspace, things would be a little easier.
As with HTML code completion, only attributes appropriate for the tag at the caret position are suggested.