just in case you did not find anything in the meantime and to add some information for others:
You can find a relatively complete example to implementing custom security here: GitHub - CustomerSecuritySample
This example utilizes a SQL database to store users, but you can change the relevant parts to use an LDAP for authentication. We omitted the implementation of the user registration, as all users already exist. There is a SOAP API for SSRS described here that allows you to add users.
Very important: you have to enter an admin user inside the rsreportserver.config:
<Security>
<Extension Name="Forms" Type="Microsoft.Samples.ReportingServices.CustomSecurity.Authorization, Microsoft.Samples.ReportingServices.CustomSecurity">
<Configuration>
<AdminConfiguration>
<UserName>Your username goes here</UserName>
</AdminConfiguration>
</Configuration>
</Extension>
</Security>
SSRS can only utilize one authentication method, so your Windows account won't do.