Call the properties/methods on the Piped Object

The short answer is no. You can't call a method like this using Pipeline. But you can surround your Get-Item invoke in parentheses and invoke it:

(Get-Item Registry::HKLM\SOFTWARE\WOW6432Node\Microsoft\Test\abc\).GetSomething()

---

If you don't want that, you could abuse the Select-Object cmdlet:

Get-Item Registry::HKLM\SOFTWARE\WOW6432Node\Microsoft\Test\abc\  | select { $_.GetSomething() }