Hi Spinx,
I honestly do not think you need to off-load any authentication onto the Load Balancers. ISA authenticates to AD and then passes those credentials to the application server, which passes them to the application.
The application should maintain the ability to authenticate to AD itself, so bypassing ISA and going directly to your application should not affect your authentication model.
ISA only acts like a firewall, so if you are going to bypass it then that is a different set of security concerns (which could be met and even exceeded by the use of the ASM if you have it).
Hope this helps.