Not sure what the end goal is but the piece you refer above is possible. The login presented by MAG is essentially a HTML form. So if this remote page can collect user credentails and do a form post to the Juniper login page cgi then it should be doable.
Note that the MAG device implements a lot of business logic using redirects, cookies, etc. So it important that the customization not break any of this logic by ignoring redirects, cookies, etc that would normnally be set if the user directly logged in.
