If you are not trying this with plugin approach.
Here is a away I do it if I were you.. edit the login FTL file to have my changes.
If I would like to create my own custom login page, then I have to override the Jive actions for login page. I'm afraid, I have never done this before.
If you just wanted to make simple changes to the text, you could use phrase substitution, or override specific i18ns.
If you would like to make phrase substitutions in login page, then phrase substitution is the best way.
++ there is no such .jspa pages in Jive. they are strut action names. So, if you wanna change the code then look for the login action and see the appropriate files where you can make changes. Hope this helps.