Is there any way to declaratively pass code-behind property values to server controls?

You're mostly right about the <%=...%> syntax. Here is an example of what happens under the hood:

<script runat="server">
    public string SomeProperty { get { return "Hello World!"; } }
</script>

<form id="form1" runat="server">
    <%= SomeProperty %>
    <div>
        <asp:Label ID="Label1" runat="server" Text="<%= SomeProperty %>"></asp:Label>
    </div>
</form>

This is parsed and the following C# code is created (I've simplified it a bit):

private Label @__BuildControlLabel1() 
{
    Label @__ctrl = new Label();
        
    this.Label1 = @__ctrl;
    @__ctrl.ApplyStyleSheetSkin(this);
    @__ctrl.ID = "Label1";
    @__ctrl.Text = "<%= SomeProperty %>";
    return @__ctrl;
}

private void @__Renderform1(HtmlTextWriter @__w, Control parameterContainer) 
{
    @__w.Write( SomeProperty );
    @__w.Write("\r\n    <div>\r\n        ");
    parameterContainer.Controls[0].RenderControl(@__w);
    @__w.Write("\r\n    </div>\r\n    ");
}

Here is an example of what happens under the hood for the <%#...%> syntax:

<script runat="server">
    public string SomeProperty { get { return "Hello World!"; } }
    protected void Page_Load(object sender, EventArgs e) { Label1.DataBind(); }
</script>

<form id="form1" runat="server">
    <div>
        <asp:Label ID="Label1" runat="server" Text="<%# SomeProperty %>"></asp:Label>
    </div>
</form>

Generates this code:

private Label @__BuildControlLabel1() 
{
    Label @__ctrl = new Label();
    
    this.Label1 = @__ctrl;
    @__ctrl.ApplyStyleSheetSkin(this);
    @__ctrl.ID = "Label1";
    @__ctrl.DataBinding += new System.EventHandler(this.@__DataBindingLabel1);
    return @__ctrl;
}

public void @__DataBindingLabel1(object sender, EventArgs e) 
{
    Label dataBindingExpressionBuilderTarget = ((Label)(sender));
    Page Container = ((Page)(dataBindingExpressionBuilderTarget.BindingContainer));
        
    dataBindingExpressionBuilderTarget.Text = System.Convert.ToString( SomeProperty , System.Globalization.CultureInfo.CurrentCulture);
}

As you can see the <%=...%> syntax can be used outside of a server control's properties to directly render the returned value. On the other hand the <%#...%> syntax generates a event handler for the DataBinding event of the label. This event sets the value of the label's property to the value of SomeProperty. The DataBinding event fires whenever the DataBind method is called which is why I added that call to the Page_Load event.
Hopefully this will help you understand the difference between them.

You could create a custom ExpressionBuilder so you use something like <%$ Code: SomeProperty %>