ADSL authentication, if I remember correctly, can be either PAP or CHAP - just like it is for a normal dialup account.
So there may be special cases whereby on their particular service, they don't record usage statistics for particular usernames, but there'd be no real way of knowing.

It sounds like their system can authenticate users by just a username/alias, or by a fully qualified hostname.
I would imagine, that the accounts are stored by hostname, and that perhaps, accounting of usage is done on an alias basis.

So if you login with your proper account name/hostname, thus bypassing your alias, then perhaps no accounting would be done.


Of course this could all be complete bull****. It really depends on the ISP themselves.