Custom patterns could be seen from UIAVerify tool (see TestStack's fork of it) and handy to use in WPF apps because pattern registration has some means to inject new pattern into existing AutomationPeer's structure.
Also was tested to some extent with WinForms.
Install-Package TestStack.UiaCustomPattersManaged -Version 0.1.2
dotnet add package TestStack.UiaCustomPattersManaged --version 0.1.2
<PackageReference Include="TestStack.UiaCustomPattersManaged" Version="0.1.2" />
paket add TestStack.UiaCustomPattersManaged --version 0.1.2
This package is not used by any popular GitHub repositories.