How to hardcode or customize the logon page to proceed for RADIUS auth without entering password in SSL/IPsec VPN client ?

from the logon page customization menu in the vpe you can remove the password field, and then set session.logon.last.password to whatever value you may need from vpe or irule.

i would first test by removing simply the password field and see if the big-ip is sending a radius request. there maybe some hidden logic where he absolutely needs a password. if yes set a dummy one and hope your radius doesn't check it.