We have Xerox iMFP. I know of no way to do what you are describing. We actually disable the personal account (since our users are required to use a cost center rather than personal accounts). When you disable the purse, the cost center defaults to the first in the list. I would be very interested in giving a user a default cost center (perhaps in the Accounts configuration). There may be a scripting option to choose a Cost Center rather than the purse, but I do not know how to do this.
I don't believe it can be set in the logon script; I believe it's inherent in the iMFP setup.
We have (mostly) Ricoh iMFPs, some with card swipes (to swipe functional account cards). 99% of our logins are hand-keyed. And we're heavy into using third-party charging; just about every user has access to a cost center. With the Ricohs, after entering the password the "personal account" button is to the right, and the list of cost centers is to the left of the screen. Either can be selected, though if there's a $0 balance in the personal account it will error, and force the user to log in again to choose a cost center.
We also have Canons (less than 10% of our Pharos fleet), and their screen walkthrough is different. I don't play with them so much, and don't have a Canon device at the shop to run through it. But I believe third-party charging is before personal account, or next to it, rather than after it in the walkthrough.
If KM's iMFP is operating that way, I think it'd have to go back to development for a change (though I hope not; we're going to probably trial a 22 ppm BizHub to see how it plays with Pharos and its iMFP, and I'd much prefer to have it work like my other two vendors' iMFPs, at least for that).