This is something I received from WBA many years ago. It still applies as nothing as changed to the Wisconsin Consumer Act since then.
1) The default rate after maturity on a loan subject to the WCA is the greater of 12% or the annual rate of finance assessed on the loan.
422.203(4)
2) You are correct in stating that bank cannot assess a delinquency/late fee if No. 1 above is in effect. 422.203(4)
3) Loans Outside of the WCA: These are loans that either governed by 138.052, chapter 428, or are "unregulated."
a) 138.052
If the loan is a first lien or first lien equivalent on the borrowers principal dwelling, then the default rate would be the contract rate in effect before default.
b) Ch. 428
Chapter 428, subchapter I, applies to a loan that is a first lien or first lien equivalent, secured by real estate, where the amount financed is $25,000 or less, and is for family, personal, or household use (that is, NOT secured by borrower's principal dwelling). This subchapter is silent with respect to default rates.
c) "Unregulated"
If the loan is not covered by Reg. Z or WI law (WCA, 138.052, ch. 428), then the contract governs as long as the terms are not unconscionable.
4) GP 458 may be used to evidence a variable rate mortgage loan provided it is not governed by the WCA. Loans exceeding $25,000 and loans secured by a first lien or first lien equivalent security interests are not governed by the WCA. The form may be used for a variable rate loan tied to an index. A non-indexed variable rate loan should be evidenced by the WBA 458 VAR Mortgage Note. The term "non-indexed" means the use of an unapproved index.