Constructs like

@{
    Name = "Staffan",
    Size = 3
}

is quite easy to infer types from, and the same is true for the conversion expression to

[pscustomobject] @{
    Name = "Staffan",
    Size = 3
}

The same mechanisms that is needed to support synthetic PSTypeNames could also be used to provide type inference for Select-Object -Property and the Group and Value propererties of the output of Group-Object

None of this is supported as of 6.1-preview 3.