Answer
Yes, ASP.NET 4.5 is supported by Plesk 11 and 12.
Below are the lists of ASP.NET versions supported by PP versions:
Plesk 9.5:
- ASP.NET 1.1.4322
- ASP.NET 2.0.50727 (.Net Framework 2.0/3.0/3.5)
- ASP.NET 4.0
Plesk 10.4:
- ASP.NET 1.1
- ASP.NET 2.0
- ASP.NET 4.0
Plesk 11:
- ASP.NET 1.1 - 4.5
Plesk 11.5:
- ASP.NET 1.1 - 4.5
Plesk 12:
- ASP.NET 1.1 - 4.5
Note #1: As .NET 4.5 Framework is iterative update to .NET 4.0, Plesk and IIS will continue displaying available version of ASP.NET as 4.0.
Note #2: See KB article #115727 for instructions on how to set up .NET 1.1 Framework on Windows 2008 with IIS7.
This information was taken from the applicable release notes: